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CHAPTER 8. PROTEIN TRANSFORMATION IN SOIL

CHAPTER 8. PROTEIN TRANSFORMATION IN SOIL

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352



MICHAEL J. LOLL AND JEAN-MARC BOLLAG



1. INTRODUCTION

Most of the nitrogen present in unfertilized soils is organic in nature. This

organic nitrogen represents an important nutrient reservoir, and a large part of it

appears to be derived from protein. In the heavily amended soils of industrialized

countries, protein plays only a secondary role as a nitrogen source, but it may be

of particular importance in some areas of the Third World where plant residues,

household wastes, and manure are often the only fertilizers available (Ruthenberg, 1976). Sulfur, as well as nitrogen, may be added to soil through the

decomposition of methionine and cysteine (Banwart and Bremner, 1976). Some

research also suggests that amino compounds contribute to the structure of soil

by their incorporation into humic acid (Swaby and Ladd, 1962; Ladd and Butler,

1966, 1975; Kononova, 1966; Felbeck, 1971; Raig ef al., 1975), from which

they may be slowly released by enzymes (Ladd and Brisbane, 1967; Sowden,

1970). After transformation they may be used as plant nutrients. The breakdown

of protein is also crucial for composting, the development of soils, the digestion

of sewage sludge, and the biodegradation of solid waste.

Protein degradation apparently is carried out mostly by microorganisms in

conjunction with mesofauna such as earthworms and insect larvae (Parsons and

Tinsley, 1975). The mineralization of protein may have a considerable effect on

soil fertility, and soil scientists have been studying microbial proteolysis in situ

and in vitro in order to understand the genesis of available soil nitrogen. In this

article, we have tried to provide an evaluation of the work done in this area and to

point out its significance in the study of soil dynamics. We have also discussed

the transformations proteins undergo in the presence of biotic and abiotic components and how these reactions may influence protein decomposition and nitrogen

availability.

II. PROTEIN SOURCES



Proteins are present in all living things and are even found in the abiotic

components of earth and water (Simonart et al., 1967; Lytle and Perdue, 1981).

Their functions are manifold, and they exist in a variety of forms, often conjugated with sugars, nucleic acids, metals, and lipids (Boulter and Derbyshire,

1977; Capaldi, 1977; Neuberger, 1978). Proteins catalyze reactions as enzymes,

act as the structural components of cells, serve as storage products, and are

involved in reactions and responses between cells. They carry oxygen and transport molecules across cell membranes (Knowles and Gutfreund, 1974; Boulter

and Derbyshire, 1977; Miller, 1978; Reid, 1978; Rupley, 1978; Warren, 1978).



PROTEIN TRANSFORMATION IN SOIL



353



Because proteins are required for so many physiological processes, they make up

a significant proportion of the dry weight of living organisms. At the death and

decomposition of microorganisms, plant and animal proteins and peptides provide the soil with significant amounts of nitrogen.

Therefore, plants are major nitrogen suppliers (Parsons and Tinsley, 1975).

Estimates of the average protein content of plants vary, the lower figures being in

the 1-15% range (Buckrnan and Brady, 1966) whereas later work (Gray and

Biddlestone, 1974) indicates that 5-40% of plant dry weight is proteinaceous.

The amount of amino nitrogen a plant contains depends on its species and stage

of development. Legumes have the most protein, about 20-30%, and soybean

percentages are sometimes as high as 38%. Cereals and most vegetables are 10

and 4% protein, respectively (Boulter and Derbyshire, 1977). Forest leaf litters

may consist of 2-15% crude protein (Gray and Biddlestone, 1974). Many seeds

are nearly all protein, and mature leaf and stem tissue is 60%peptide and protein

matter (Parsons and Tinsley, 1975). With water, proteins comprise most of the

protoplasm of higher plants and participate in the synthesis of cellular components. They are not found in the cell walls of plants, and they appear to be

primarily functional (Black, 1968) rather than structural.

Animals are next in importance in the contribution of protein to the soil

nitrogen pool. It has been estimated that 80% of muscle tissue is made of protein

and nucleic acids (Gray and Williams, 1971). Although they contain more protein than plants, animals ordinarily represent a smaller proportion of the biomass

of most habitats. On grazing land the excreta of sheep and cattle affect nitrogen

content over long periods (Parson and Tinsley, 1975); protein accounts for

5-30% of the weight of manure (Gray and Biddlestone, 1974). In areas with

large bird populations, keratin from feathers may furnish soil with a-amino

nitrogen. Insects and arthropods, whose exoskeletons are made of chitin-protein

and carbohydrate-protein complexes (Hunt, 1970), have also been implicated as

potential nitrogen sources (Parsons and Tinsley, 1975).

Bacteria, actinomycetes, fungi, and algae account for the rest of the protein

found in soil. The quantities of protein that these organisms deposit in the earth

are considered negligible by some authors. Using data from Stockli (1946),

Parsons and Tinsley (1975) point out that of the 2 kg/m2 of microbial biomass in

the Swiss meadow, only 1% is nitrogen. Assuming that protein is approximately

15% nitrogen, the amount of protein in the meadow soil is only 134 g/m2.

However, biomass determinations are subject to various errors, and other authors

have given much higher values for the amount of microorganisms in soil. Estimates have ranged from 300 to 6400 pounds of bacterial tissue per acre-furrow

slice (Clark, 1967), and radiolabeled bacterial amino acids have been isolated

from soil by Wagner and Mutatker (1968). Therefore it is possible that microbes

may account for a large part of the soil nitrogen.



354



MICHAEL J. LOLL AND JEAN-MARC BOLLAG



I11. PROTEOLYTIC MICROORGANISMS

Soil is a “sink” in which proteins from plants and animals accumulate, and as

such it is a site of intense protein and peptide hydrolysis. This accumulation

process varies with the season and ecosystem and is determined by soil characteristics, the kind of vegetation covering the soil, and the nature of the soil

microbial community. Protein decomposition is carried out by the soil microflora, which can utilize the resulting amino acids as carbon and nitrogen sources.

Indeed, large numbers of bacteria, actinomycetes, and fungi are involved in the

biodegradation, and as many as 105-107proteolytic microbes per gram of soil

have been isolated from surface horizons (Alexander, 1977). Similar concentrations have been observed in sewage waste (Hobson, 1973). Protein decomposers

may comprise 22-89% of the total soil population (Hankin and Hill, 1978)and

are found in a wide range of environments. Rice paddies (Kobayashi et al., 1967;

Ishizawa er al., 1969),deserts (O’Brien, 1978),polders reclaimed from the sea

(van Schreven and Harmsen, 1968),tundras (Dunican and Rosswall, 1974),sand

dunes, salt marshes (Pugh and Mathison, 1962),meadows (Chmel and Vlacilikova, 1975),field soils (Lajudie and Chalvignac, 1950),estuaries (Sizemore and

Stevenson, 1974), sewage sludge (Hobson et al., 1974;Cox, 1978) and the

rumens of sheep and cattle (Blackburn and Hobson, 1962)all support proteolytic

populations of various sorts.

Proteases and peptidases, synthesized by soil microorganisms are the catalysts

responsible for breaking down proteins. Many of these enzymes are exocellular,

as a large number of native proteins are too large to be absorbed by living cells.

Proteases released from cells fragment protein into smaller, membrane-permeable peptides and amino acids which microbes can metabolize. Degradation results from two modes of hydrolysis: (1) attack on the terminal amino acid of the

peptide chain, which is by exopeptidases; and (2) attack on nonterminal peptide

bonds by endopeptidases (Alexander, 1977). The amino acids can be metabolized to ammonia and carbon dioxide, and five different reactions have been

identified.

A. Hydrolytic deamination

RCHzNHzCOOH + H20 + RCHOHCOOH + N H 3

RCO + HCOOH + N H 3

RCHzOH + COz + NH3



B . Reductive deamination

RCHNHZCOOH + 2 H --* RCHZCOOH + NHs



C. Oxidative deamination

RCHNHzCOOH



+ 4 0 2 + RCOCOOH + NHs



PROTEIN TRANSFORMATION IN SOIL



355



D. Ammonia removal

RCHNHzCOOH + RHC = CHCOOH + NH3



E. Decarboxylation

RCHNHzCOOH + RCHzNHz



+ COz



Intracellular enzymes from roots and dead plants and animals could also

contribute to protein hydrolysis in soils. In fact, Ladd (1972) produced soil

extracts having characteristics akin to plant and animal proteases, and proteolytic

activity has been shown to increase when soil is incubated with plant residues

(Ilyaletdinov et al., 1972). However, several investigators have been unable to

detect proteases being exuded from plant roots (Estermann and McLaren, 1961;

Chang and Bandurski, 1964; Vagnerova and Macura, 1974a). Therefore it is

generally assumed that the living microflora are responsible for most of the

degradative potential of a soil.

Enzymologists have classified proteases, microbial and otherwise, into four

basic groups, as shown in Table I. The substrate specificities vary, but in general

tend to be rather wide. Proteases that act on high molecular weight proteins are

generally exocellular, whereas intracellular hydrolases act on low molecular

weight proteins (Ladd and Butler, 1975). In some fungi and bacteria proteases

are inducible, and although the pure culture studies done on the subject may not

present an accurate model of what happens in soil, they give us some idea of the

potential mechanisms that may be at work in the environment. Gill and Modi

(1981) induced the production of exocellular proteases in Aspergillus nidulans by

supplementing media with cooked egg white. Proteolytic activity in a species of

Serraria increased when it was grown with bovine serum albumin, a-lactalbumin, or peptone (Murakami et al., 1969); peptides have been used by

Japanese scientists to induce proteases in thermophilic species of Streptomyces

(Mizusawa et al., 1966).

Thousands of species of soil microorganisms produce proteases and only a few

of the more prominent genera are listed in Table 11. Many of these microbes are

not found exclusively in soils, but are also found in fresh water and marine

environments, feces, wounds, and the gastrointestinal tracts of humans and

animals. Bacteria, actinomycetes, and fungi can adapt to various ecosystems,

and so-called indigenous soil populations may arise from a variety of sources.

The Occurrence of a proteolytic microflora depends on the following environmental conditions.

A. HABITATS



Agricultural soils, such as those of orchards, pastures (Hankin et al., 1974),

and plantations (Visser and Banage, 1973), often contain more protein decom-



Table I

CharacteristirS and Clapsitlcrrtion of Proteaseso



Enzyme



pH optima



Molecular

weight



Metal

requirement



Esterase

activity



-



Limited



Acid proteases

Thiol proteases



4 ~ 8



35,000

20,000-50,000



Metalloproteases



7-8



35,Ooo-45,Ooo



1-5



-



Zn, Mg,Co,



Inhibitors



-



Diazoketones

Iodine, iodoacetate, organic

~ K W H202

,

EDTA, 0-phenanthrolie



Extensive



DFP, sarin, PMSF, DFCC



-



Fe, Mn, Ni

Alkaline or

serine proteases



9-11



26,000-34,000



-



“Data from Cunningham (1%5), Keay (1971), Mntsubara and Feder (1971), Priest (1977). and Walsh (1975).



Examples

Pepsin

Papain, ficin, bromelin,

cathepsin

Carboxypeptidase A,

themolysin

Trypsin, chymotrypsin, thrombin, subtilisin, elastase



357



PROTEIN TRANSFORMATION IN SOIL



Table II

Proteolytic Microorganisms

Genus

Actinomycetes

Acrinomyces

Micromonospora

Nocardia

Streptomyces



Reference



Clark and Paul (1970)

Ishizawa er al. (1969)

Fergus (1964), Clark and Paul (1970)

Fergus (1964), Clark and Paul (1970), Eklund et al. (1971), Knosel

(1974)



Thermoacrinomyces

Thermonospora

Bacteria

Achromobacrer

Arthrobacter

Bacillus

Bacreriodes

Clostridium

Corynebacrerium

Fluvobacrerium

Micrococcus

Peptococcus

Proreus

Pseudomonas

Sarcina

Serratia

Staphylococcus

Sfreptococcus

Fungi

Alrernaria

Amuuroascus

Anrjriopsis

Arthroderma

Aspergillus

Auxarthon

Cephalosporium

Chaeromium

Chrysosporium

Ctenomyces

Cunninghamella

Curvularia



Fergus (1964)

Fergus (1964)

Pochon and Chalvignac (1952), Ueda and Earle (1972)

Clark and Paul (1970)

Pochon and Tchan (1947), Pochon and Chalvignac (1952); Appleby

(1955), Clark and Paul (1970)

Siebert and Toerien (1969)

Pochon and Tchan (1947), Appleby (1955), Siebert and Toerien

(1%9), Cox (1978)

Appleby (1955), Clark and Paul (1970)

Appleby (1955), Clark and Paul (1970)

Clark and Paul (1970)

Siebert and Toerien (1969)

Pochon and Tchan (1947), Pochon and Chalvignac (1952); Appleby

(1955), Clark and Paul (1970)

Pochon and Tchan (1947), Kolesnikova er al. (1972), Ueda and

Earle (1972)

Pochon and Tchan (1947)

Clark and Paul (1970)

Siebert and Toerien (1969)

Clark and Paul (1970)

Griffin (1%0), Prudlov et al. (1973)

Bohme and Ziegler (1969)

Bohme and Ziegler (1969), Chmel and Vlacilikova (1975)

Griffin (1960, 1972); Bohme and Ziegler (1969); Chmel and

Vlacilikova (1975)

Pugh and Mathison (1962)

Pugh and Mathison (1962), Fergus (1964), Clark and Paul (1970)

Bohme and Ziegler (1969)

Clark and Paul (1970)

Griffin (1960), Fergus (1964), Ong and Gaucher (1973)

Bohme and Ziegler (1969), Chmel and Vlacilikova (1975)

Pugh and Mathison (1962), Bohme and Ziegler (1969), Chmel and

Vlacilikova (1975)

Griffin (1960)

~~



~



(Continued)



MICHAEL J. LOLL AND JEAN-MARC B O U A G



358



Table II Continued

Genus



~



Diheterospora

Epicoccum

Fusarium

Ganoderma

Gliocladium

Gymnoascus

Helminthosporium

Humicola

Keratinomyces

Lentinus

Malbranchea

Martierella

Microsporon

Mucor

Nannizia

Paecilomyces

Penicillium

Phoma

Polyporus

Polystictus

Pyrenochaeta

Rhizopus

Stilbella

Talaromyces

Thielavia

Trametes

Trichoderma

Trichophyton



Reference

Griffin (1960)

Chmel and Vlacilikova (1975)

Griffin (1960)

Griffin (1960), Rudlov et al. (1973)

Das et al. (1979)

Griffin (1960)

Griffin (1960)

Griffin (1960)

Griffin (1960), Fergus (1964), Ong and Gaucher (1973)

Griffin (1960)

Das et al. (1979)

Ong and Gaucher (1973)

Griffin (1960)

Bohme and Ziegler (1969)

Griffin (1960), Clark and Paul (1970)

Pugh and Mathison (1%2), Bohme and Ziegler (1969), Griffin

(1972), Chmel and Vlacilikova (1975)

Griffin (1960)

Griffin (1960), Clark and Paul (1970), Ong and Gaucher (1973)

Griffin (1960)

Das et al. (1979)

Clark and Paul (1970)

Griffin (1960)

Clark and Paul (1970)

Fergus (1964)

Fergus (1964), Ong and Gaucher (1973)

Griffin (1960)

Das et al. (1979)

Griffin (1960), Rodriguez-Kabana et al. (1978)

Griffin (1960). Bohme and Ziegler (1%9), Chmel and Vlacilikova

( 1975)



posers than many virgin soils. Swamps (Visser and Banage, 1973) and tidal

marshes (Cahenzli and Staffeldt, 1976) are other sites where proteolytic microbes may be in abundance. These are areas where there is much organic matter

from plant and marine animal residues, and the addition of protein to soil appears

to stimulate microfloral populations (Holding ef al., 1965).

The plant rhizosphere has a pronounced influence upon soil proteolysis. Plant

roots exude a number of nutrients, including amino acids and peptides, which

promote the proliferation of microorganisms. The rhizospheric effect varies with

the particular plant. More than 40% of the bacteria isolated from the roots of

wheat, beans, peas, cucumbers, and barley have been found to hydrolyze protein. The percentages for other agronomic plants have been a little less, with 18%



PROTEIN TRANSFORMATION IN SOIL



359



for corn, 31% for clover, 25% for lettuce, 34% for red pepper, and 39% for

tomato roots (Vagnerova and Macura, 1974~).Katznelson and Rouatt (1957)

reported that more ammonifiers are present in the rhizosphere soils of wheat, rye,

barley, and oats than in nonrhizosphere soils. Conversely, a decrease in the

number of protein decomposers has been observed around the roots of lodgepole

pines (Dangerfield et al., 1978). Apparently, isolates from the roots of these

pines are less proteolytically active than isolates from the rhizospheres of other

plants.

B. PH



The role of pH in regulating microbial growth is not clear, at least for proteolytic species. The commonplace is that fungi (because of their tolerance of low pH)

are the major users of protein in acid forest soils. Bacteria and actinomycetes are

thought to be more prevalent in neutral and alkaline regimes (Chalvignac, 1953;

Alexander, 1977), but this is not true in all cases. Hankin and Hill (1978) were

unable to correlate bacterial numbers with pH. Many keratinophilic fungi prefer

weakly acid to weakly alkaline soils (Bohme and Ziegler, 1969), although there

are species which are prevalent in acid bogs (Griffin, 1972). Holding er al.

(1965) observed bacterial proliferation when they added nutrients to soils with

pH values as low as 3.7. They suggested that most soils, given the proper

conditions for growth and small fungal populations, will support bacteria regardless of pH. For any broad class of organisms there are wide ranges of tolerance,

and it is not astonishing to see differences in pH sensitivity from one species to

another.

C. Son, ATMOSPHERE



Whereas early work (Chalvignac, 1953) emphasized the importance of aerobes in the mineralization of protein, it is now known that both aerobic and

anaerobic microorganisms are involved in the process. Aerobic metabolism is

predominant at the soil surface and in the litter layers; anaerobic degradation

occurs in deep horizons and waterlogged soils. Proteolytic anaerobes are common in sewage sludge digesters (Siebart and Toerien, 1969; Hobson er al.,

1974), and some of these come from soil (Cox, 1978). Aerobic protein decomposers appear to be more numerous in the environment and proteolysis may be

inhibited without oxygen (Sizemore and Stevenson, 1974). Keratinophilic fungi

are concentrated at the soil surface and become fewer down the soil profile

(Chmel and Vlacilikova, 1975). Using samples taken from a beach, Sizemore

and Stevenson (1974) isolated a number of saprophytes, 75% of which were

proteolytic when grown under oxygen.



360



MICHAEL J. LOLL AND JEAN-MARC BOUAG



Volatile substances produced from decaying plants can inhibit the growth of

pmteindegrading soil bacteria. Lucerne hay, which releases acetaldehyde as it

decomposes, reduces the number of protein hydrolyzers when incubated with

soil for long periods of time (van Schreven, 1972).

D. TEMPERATURE



In cold soils, psychrophilic bacteria seem to have a significant role in the

biodegradation of protein (Stefaniak, 1972), but the actinomycete population

decreases with decreasing temperature. Low temperatures inhibit, but do not

stop, the growth of proteolytic bacteria (Stefaniak, 1968). Thermophilic fungi

like Penicillium duponti and Malbranchea pulchella var. sulfurea are capable of

pmtease synthesis (Ong and Gaucher, 1973) and are probably active during the

summer and in compost heaps.



E. SUEISTRATE

Different types of proteins have varying effects on the composition of the

microflora in soil, and a particular substrate may favor the survival of a particular

type of organism. Putyatina (1966) found that vegetative albumin, yeast protein,

and casein promoted the development of mycobacteria; spore-forming bacteria

developed when soil was amended with animal albumin and glutenin.

F. SALTCONCENTRATION



Proteolytic bacteria from estuaries have been inhibited by the presence of salt

(Stefani and Sequi, 1978). Mineral fertilizers could increase the salt content of

soil if used improperly, with a consequent reduction of the saprophytic

community.

G. SUCCESSION



Ecological succession has been observed on protein substrates added to soils.

Pochon and Chalvignac (1952) incubated bits of liver and kidney with soil and

then examined the populations that developed. Their results showed a gradual

change with time from nonsporulating gram-positive bacteria to sporulating

gram-negative ones. At the end of 18 days of incubation, autolysis took place as

the last of the added protein was metabolized. As they isolated few actinomycetes and no fungi, they assumed that these organisms were not involved in



PROTEIN TRANSFORMATION IN SOIL



36 1



proteolysis. However, this may have been because of the inadequacies of their

media for selecting these groups of the microbial community. Okafor (1966) has

proposed another scheme of succession. When proteinaceous insect wings were

added to soil, proteolytic fungi were among the first colonizers. The subsequent

appearance of bacteria on the wings was related to the utilization of substrates

other than protein.

The competitive abilities of the colonizing organisms are also important, and

in the breakdown of sterile hair the less specialized fungi develop first. These are

often cellulolytic organisms, such as species of Humicola or Chuetomium, which

are capable of using polysaccharides. Sometime afterward keratinophilic fungi,

which are more selective in their substrate utilization, begin to grow (Griffin,

1960). The same study mentions that the previous history of the soil under

investigation is fundamental in determining the course of colonization. Soils

having large inputs of keratin and manure from birds tend to be good sources of

keratinophilic microbes.

The isolation and identification of proteolytic microbes is necessary for understanding the degradation of protein in soil. However, isolations are not wholly

representative of the active population and sometimes cells ordinarily dormant in

soils are activated when introduced into nutrient-rich media. The metabolic

properties of a cell may change when transferred from soil to media, and certain

hydrolytic activities occurring in pure cultures may be absent under natural

conditions. Enzyme synthesis to some extent is a function of the age of a cell or

mycelium, and large numbers of bacteria do not mean a high activity. Microbiologists have sought to minimize interferences of this type by studying the

enzymatic activity of soil as a whole. This approach, based on extracting and

assaying soil proteases in order to measure the proteolytic potential, is thought to

serve as an assay for protein degradation in soil.



IV. CHARACTERISTICS OF PROTEOLYTIC

ENZYMES IN SOILS



Several investigators have tried to characterize proteolytically active extracts

from soil, but such analyses have been complicated by the association of proteases with soil constituents. These complexes are thought to prolong the survival of enzymes in the environment, and in some cases the protein and soil

components appear to be bound together quite strongly. Complexation reactions

make the purification and characterization of soil enzymes so difficult that no one

has yet made a complete purification of a protein from soil (Tinsley and Zin,

1954; Jenkinson and Tinsley, 1959; Simonart et al., 1967; Biederbeck and Paul,



362



MICHAEL J. LOLL AND JEAN-MARC BOLLAG



1973; Mayaudon and Sarkar, 1974). Analysis is also hindered by the fact that

soil proteases originate from a variety of sources. Proteases come from plants,

animals, and microbes, and represent a heterogeneous mixture of enzymes having different molecular weights, structures, substrate specificities, etc. The type

of protease extracted depends on the proteases’ stability, their association with

humic material, and their chemical nature, and on the kind of soil in which they

are found and the extractant used. A particular extractant may remove several

proteases from a sample simultaneously. In spite of these problems, however,

work in this area has given us some insight into the reactivity and composition of

“humo-enzymes” and how they act upon substrates in the soil.

Determining the exact composition of soil proteases is a formidable task. They

are probably bound in some way to organic and inorganic substances from

humus, clays, and minerals. In fact, no complete analysis has been done on

proteolytic soil extracts; they do, however, appear to contain significant amounts

of humus and carbohydrates. Sarkar er al. (1980) speculate that a number of

proteinases are glycoproteins in soil and that carbohydrates stabilize soil enzymes. Humic acids also have a protective effect on proteases. Pronase, when

complexed with humic acid analogs prepared from p-benzoquinone, had higher

temperaturn and pH optima for activity and was more thennostable, although less

active (Rowel1 er al.. 1973), than its native form. Nothing is known about the

cofactor requirements or structures of the active components of soil extracts.

The activities of soil proteases are greatly affected by changes in temperature

and pH. Working with Tris-borate extracts of some Australian soils, Ladd

(1972) found that the optimal pH for protease activity was between 7.0 and 8.1

when 2-phenylalanyl leucine was used as the substrate. This range may include

the optima of neutral and alkaline soil proteases, such as those described by

Ambroz (1970) in central European chernozems and rendzinas. He observed

maximal activity for the neutral proteases at a pH of 6.5, whereas the alkaline

proteases showed their highest activities at 8.5. A multienzyme complex extracted from soil with EDTA also had its maximum caseiolytic activity at pH 8.5.

Proteinase-like extracts are heat labile, and Mayaudon and co-workers (1975)

reported a complete loss of activity in one extract when it was heated to 75°C.

From 20 or 30”C, activity increases with temperature and reaches a peak around

50-60°C, after which it drops off rapidly (Ladd, 1972). The activation energy

for one proteolytic extract has been estimated to be 20 kcal (Mayaudon et al.,

1975).

Proteolytic soil extracts appear to be relatively stable to a number of environmental stresses. When lyophilized, they can be stored at 1-25°C for prolonged

periods of time without denaturing. Incubation at higher temperatures, however,

will result in inactivation, and as much as 60% of the activity may be destroyed

over a 24-hr period when the extracts are kept at 50°C. Proteolytic extracts can



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