Alloxan enzyme inhibition

Antioxidant enzymes also protect against free radical-mediated DNA degradation. For example, SOD and catalase as well as PARS inhibitors suppressed alloxan- and streptozotocin-induced islet DNA strand breaks [122], Uric acid inhibited single-strand DNA breaks induced... 

An additional crucial piece of information emerges from the alloxan-thine study (24). Thus, it was shown that one alloxanthine binds to the enzyme per active molybdenum site. This result clearly implies that the molybdenum site is mononuclear. If a dinuclear site were involved, then it would be unlikely to require two alloxanthine molecules for inhibition and would be expected to be at least partially inhibited with one alloxan-thine/two molybdenum. Also, a difference in binding constant would be expected for the second compared with the first bound alloxanthine, but none is found. This result, coupled with the lack of evidence for Mo(V)-Mo(V) spin-spin interactions in the EPR spectra, clearly implicates a mononuclear site, and it would seem that xanthine oxidase possesses two full catalytic units, each containing one molybdenum, one flavin, and two Fe2S2 units (20). Other molybdenum oxidases also contain paired prosthetic groups and subunits, and it is likely that they each have two catalytic units per molecule. 

Pro-drug (a suicide substrate) converted by xanthine oxidase, forming alloxan-thine, which inhibits the enzyme — 4> purine metabolism —uric acid. Also used in cancer chemotherapy and radiation therapy. 

Iodine is a far more eflFective inhibitor than other oxidizing agents, e.g. alloxan or ferricyanide, and it is therefore very likely that iodine inhibition is not a result of oxidation but of direct combination with the enzyme, possibly with tyrosine residues (280). 

The effects of anoxia on glucose phosphorylation in hearts from alloxan-diabetic rats have been studied by Morgan et al. (1959b). Anoxia accelerated glucose phosphorylation in the diabetic heart as it does in the normal heart, and the rate attained was approximately 80% of that of the normal heart under anaerobic conditions. Sinc.e glucose phosphorylation was stimulated by anoxia, it seems reasonable to regard the phosphorylation system principally as inhibited rather than deficient in enzyme content. 

Studying the effect of diabetic plasma on hexokinase activity of rat brain, Weil-Malherbe [129] observed an inhibition of enzyme activity in half of the cases, but insulin injection successfully eliminated the inhibitory effect. Similarly, Bornstein and Park [130] observed the inhibition of glucose uptake in rat diaphragm incubated in the presence of sera obtained from alloxan-diabetic rats. The inhibitory properties disappeared either by adding insulin to the incubation medium or by simultaneous hypophysectomy and adrenalectomy of the diabetic animals. 

Viewing the fact that only a portion (but not all of the acid phosphatase) of the lysosomal fraction is readily released upon physical disruption of the lysosomal membrane by freezing and thawing or by hypoos-motic pressure, Baccino et al. (1971) suggested that at least two varieties of acid phosphatase were associated with the lysosomal fraction, the first being readily, and the other not readily, dissociable from lysosomal structures. This interpretation coincides with the earlier observation of Sloat and Allen (1969) who showed two varieties of acid phosphatase associated with lysosomal fractions of rat liver. One form is readily released after physical disruption of lysosomal fractions, the other form is associated with the lysosomal membrane and became soluble only with 5% Triton X—100 treatment. This membrane-associated enzyme accounted for 40% of the total lysosomal acid phosphatase, is heat-stable, and can be separated from the soluble form by electrophoresis. But these two enzymes have similar pH optima and a common response to inhibitions by L-tartrate, fluoride, alloxan, and formaldehyde. 

The Effect of Insulin on Individual Enzyme Systems. The pioneering work in this field of approach has been carried out by the Coris and their co-workers. They observed that in muscle extracts of alloxan-diabetic rats, of thirty animals, fifteen exhibited hexokinase activity which was inhibited 21% to 76% by the addition of an adrenal cortex extract, whereas the remaining fifteen showed less than 15% inhibition. This inhibition could not be observed with extracts of normal rat muscles. Brain and muscle hexokinase were inhibited by the addition of certain... 

Sotalol, a 8-adrenergic receptor blocker devoid of these membrane effects, did not inhibit AC under similar conditions. Alloxan and ethacrynic acid apparently act via AC inhibition in the pancreas and kidney, respectively. There are indications from studies with brain tissue in vitro that cobalt ions may inhibit guanylyl cyclase (GC) activity . In addition to either stimulating or inhibiting cyclase activity, blockade of enzyme stimulation by natural stimulants represents a major site for the action... 

Numa et al. (1961), Wieland et al. (1963), and Korchak and Masoro (1962) have demonstrated decreased acetyl-CoA carboxylase activity in liver following starvation or alloxan diabetes. Liver and adipose tissue fatty acid synthesis is virtually arrested in these circumstances. Korchak and Masoro (1962) noted that after a 24-hour fast, the rate-limiting enzyme, acetyl-CoA carboxylase, fell to 50% of its control activity while at the same time there was a 99% depression in fatty acid synthesis by the intact liver cell. They interpreted this to mean that the inhibition of lipogenesis cannot be entirely accounted for by... 

Although structure-function relationships of G-6-Pase are of interest in themselves, it is important to point out that the alterations of activity seen in vitro as a result of treatment with phospholipases have functional counterparts in vivo. Already mentioned above is the Kq-s-p change in fasted or alloxan-diabetic animals (Segal and Washko, 1959 Nordlie et al., 1968). In some settings it is also possible to show that apparent increases in the activity of G-6-Pase at Vmax are not due to the synthesis of new enzyme. In rats treated with the corticosteroid triamcinolone, for example, G-6-Pase activity increases, but this increase is not inhibited by prior administration of actinomycin D (Arion and Nordlie, 1967). On the other hand, whereas in vitro treatment of micro-somes with deoxycholate increases the activity of G-6-Pase in micro-somes from control animals by about 50 percent, the increase on deoxycholate treatment of microsomes from animals treated with acti-... 

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