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SUMMARY: The stimulatory action of zinc on aflatoxin production by Aspergillus parasiticus NRRL3240 has been investigated by studying the levels of tricarboxylic acid (TCA) cycle intermediates and related enzymes in the fungal mycelium. During the stationary phase of growth, the levels of α-keto acids declined in zinc-sufficient cultures compared with those in zinc-deficient cultures. TCA cycle enzymes did not show any significant changes due to zinc availability. In zinc-deficient cultures, enzymes of the TCA cycle had maximum activity on the fourth day, after which their activity declined. In zinc-sufficient cultures, some enzymes showed maximum activity on the fourth day, others on the second day.

Enzymes of the glycogen cycle, namely, UDPG-glycogen transglucosylase, phosphorylase, phosphoglucomutase, and glucose-6-phosphatase, were assayed in the liver of normal and tuberculous guinea pigs. A significant decrease in the activities of the first three enzymes and increase in that of the last one were noted. The decrease in the activity of UDPG-glycogen transglucosylase was of significant magnitude to account for the depletion of the hepatic glycogen in tuberculous guinea pigs.

Highly purified sheep lung cyclic-3',5'-nucleotide phosphodiesterase was sensitive to Ca2+/EGTA but insensitive to exogenous calmodulin. The Ca2+-sensitivity was inhibited by trifluoperazine. Heat-treated enzyme could activate a calmodulin-deficient phosphodiesterase, suggesting the presence of endogenous calmodulin in sheep lung cyclic-3',5'-nucleotide phosphodiesterase, possibly associated with the enzyme in a Ca2+-independent manner.

Ca2+-sensitivity of sheep lung cyclic-3',5'-nucleotide phosphodiesterase is provided by endogenous tightly bound calmodulin. The calcium sensitivity of a highly purified enzyme was desensitized by increasing the assay temperature. It could also be desensitized to Ca2+-activation by thiols such as dithiothreitol. The thiol-induced desensitization could be partially reversed by dialysis and almost completely reversed by dilution. The results presented in this paper indicate that thiols are possibly involved in the interaction of calmodulin with cyclic-3',5'-nucleotide phosphodiesterase. This is the first report on temperature and thiol-induced desensitization of Ca2+-sensitivity of a cyclic-3',5'-nucleotide phosphodiesterase.