The regulation of apoenzyme levels by coenzymes and hormones

Evidence has been obtained that coenzyme levels, in addition to regulating the activity of existing enzyme, may influence in vivo the amount of protein moiety of appropriate enzyme systems. Pyridoxine administration to rats (intact or adrenalectomized) causes, within 4 hr, an approximately 200 per cent increase in the level of liver tyrosine transaminase. This change reflects an increase in the concentration of the apoenzyme since the activity both before and after pyridoxine treatment was measured in the presence of excess added pyridoxal phosphate. Furthermore, puromycin, an inhibitor of protein synthesis, interferes with this cofactor-induced rise in vivo. The effectiveness of tryptophan analogues as inducers of rat liver tryptophan pyrrolase correlates with their ability to raise the holoto apotryptophan pyrrolase ratio. This provides further support for the hypothesis regarding the role of the heme coenzyme in the regulation of the synthesis of this enzyme. The possible mechanisms involved in cofactor induction are discussed and contrasted with the hormonal regulation of enzyme levels.