Enzymes adaptive

Table 19-1. Regulatory and adaptive enzymes of the rat (mainly liver).

Both dehydrogenases of the pentose phosphate pathway can be classified as adaptive enzymes, since they increase in activity in the well-fed animal and when insulin is given to a diabetic animal. Activity is low in diabetes or starvation. Malic enzyme and ATP-citrate lyase behave similarly, indicating that these two enzymes are involved in lipogenesis rather than gluconeogenesis (Chapter 21). 

The Fatty Acid Synthase Complex Acetyl-CoA Carboxylase Are Adaptive Enzymes... 

M10. Monod, J., Jacob, F., and Gros, F., Structural and rate-determining factors in the biosynthesis of adaptive enzymes. Biochem. Soc. Symposia (Cambridge, Engl.) No. 21, 104-132 (1962). 

In addition there is other evidence pointing to the fact that the same enzyme is involved in reactions with both D-fructose and L-arabinose. First, the relative rates of reaction with D-fructose and L-arabinose, respectively, remain constant after partial inactivation of the enzyme by heat. Second, the enzyme catalyzing both reactions is produced to a marked extent when sucrose is used as substrate for the growth of the organisms, but not when D-glucose or L-arabinose is used sucrose phos-phorylase is an adaptive enzyme. Third, on fractionation of the enzyme preparation with various concentrations of ammonium sulfate, the relative activities of the fractions are the same for both sugars. These observations indicate not only that the same enzyme is involved in both reactions but also that no additional enzyme is required for the formation of D-glucosyl-L-arabinose. 

They stated further that, the new adaptive enzyme catalyzing Reaction 3 appears to be similar to the malic enzyme of pigeon liver, although strictly DPN (instead of TPN)-specific. The coenzyme specificity explains the ready occurrence of Reaction 1. Therefore, the authors showed that exogenous NAD was required for the overall reaction (malic acid -> lactic acid), but because this activity was measured manometrically, they never demonstrated the formation of reduced NAD. Similarly, they did not attempt to show that pyruvic acid was the intermediate between L-malic acid and lactic acid. Instead, the formation of pyruvic acid was inferred from the NAD requirement and because the malic acid dissimilation activity remained constant during purification while the lactate dehydrogenase activity decreased (14). In fact, attempts to show any appreciable amounts of pyruvic acid intermediate failed (22). 

It was reported (14) that the adaptive enzyme from Lactobacillus plantarum could decarboxylate oxaloacetic acid as well as malic acid. However, in the same organism, Nathan (30) carried this work further and showed that the oxaloacetate decarboxylase activity is not related at all to the malic acid-lactic acid transformation activity. She based this conclusion on the ability of malic and oxaloacetic acids to induce oxaloacetate decarboxylase activity as well as malic enzyme activity. In her words,... 

The first replicative units must have possessed considerably less information than the RNA viruses, which work with an optimized RNA-copying machinery. In the absence of efficiently adapted enzymes the accuracy of reproduction depends solely on the stability of the base pairs. Under these conditions the GC pair has a selective advantage over the AU pair of a factor of about 10. Model experiments show that for GC-rich polynucleotides the error rate per nucleotide can hardly be reduced below a value of 10-2. The first genes must accordingly have been polynucleotides with a chain length around 100 bases or less. 

All these approaches have been used to alter protein function, to increase the activity or solubility of proteins, or to adapt enzymes for industrial applications. The goal of artificial man-made proteins with tailor-made activities is, however, still far away and none of the currently existing approaches provides the ultimate solution to the directed evolution of proteins. Nevertheless, numerous examples of successfully altered and improved proteins clearly show the power of directed evolution for protein design. 

It has been known for some time that bacteria can alter their enzyme composition in response to changes in their environment. Early studies demonstrated that certain enzymes in microorganisms were produced in significant amounts only when their substrates were present in the culture medium. Such enzymes were considered a special group and were first called "adaptive" enzymes now they are called inducible enzymes because they are formed de novo from amino acids rather than from protein precursors. Opposed to the inducible enzymes were the constitutive enzymes, which were formed in constant amounts regardless of the metabolic state of the organism. 

Adaptive Enzyme Theory. The aliesterases are largely found in the microsomes of rat liver cells (44). Recently Hart and Fouts (51,52, 67-69) have presented evidence that in vivo administration of chlordan or chemically related DDT stimulates the activity of hepatic microsomal drug-metabolizing enzymes, as evidenced by proliferation of smooth-surfaced endoplasmic reticulum (SER) which was first noted with phenobarbital. Several reviews of hepatic drug metabolism... 

These phenomena may be explained in terms of hepatic lysosomal membrane change induced by the chemicals (suggested by recent work with phenothiazine derivatives), or in terms of stress and adrenocortical intervention, or of adaptive enzyme power of the liver. 

Yamanaka and co-workers (364-366) have crystallized a cytochrome oxidase from P. aeruginosa which oxidizes Pseudomonas ferrocytochrome c-551. It is also capable of nitrite reduction with a turnover number of 4000 moles nitrite reduced under anaerobic conditions to nitric oxide per minute at 37°. It is an adaptive enzyme, nitrate being essential for its biosynthesis. The enzyme has a molecular weight of 120,000, with two subunits of equivalent molecular weight, 2 heme c and 2 heme d groups per mole (Fig. 38) (366a). Nitrite reductase activity is 94% inhibited by 8 X 10 M KCN, but only by CO. The lack of CO inhibition appears to be related to the fact that the enzyme has a greater affinity for nitrite than for carbon monoxide. 

That hydroxylamine might not be an obligatory intermediate, or occur as a free intermediate, in the reduction of nitrite to ammonia is suggested by the properties of nitrite reductases of Azotobacter chroococcum and Escherichia coli. The former is an adaptive enzyme, the formation of which requires nitrate or nitrite in the culture (31,2). It is FAD-depen-dent and presumably contains metals and p-mercuribenzoate inhibitable... 

The production of D-xylanase activity by fungi cultured on cellulose may also be due to the action of a cellulase component having multisubstrate activity. Cellulases have always been reported to be adaptive enzymes,226 and the multiplicity of such enzymes may imply that this pseudo-xylanase activity is produced constitutively when fungi are grown on cellulose as the sole source of carbon. Cellulases of this type have been shown203 also to attack D-xylan even when they are highly purified (for example, cellulase F-2 from Trichoderma viride203). 

Hydroxykynurenine excretion in pathological states was also first reported by Japanese workers (573), who identified it as the substance causing the diazo reaction and the Weiss urochromogen reaction in urines from cases of severe tuberculosis. This was confirmed in the author s laboratory (178), where it was also shown that the excretion is unrelated to tuberculosis as such. Hydroxykynurenine excretion occurs in a large proportion of patients with fevers of varying etiology and is in all probability due to the increased rate of breakdown of body proteins in fever. Presumably the protein breakdown induces an adaptive increase in tryptophan peroxidase-oxidase, and the capacity of the available kynureninase, which comes later in the metabolic chain and is not an adaptive enzyme (480), is exceeded. 

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