Stabilization of coenzyme binding by conjugated steroids and carboxylic acids

Estradiol disulfate and diethylstilbestrol disulfate, which interfere with the binding of pyridoxal phosphate to kynurenine apotransaminase and protect the apoenzyme from proteolytic inactivation, the dissociation of pyridoxal phosphate from the transaminase in freshly homogenized rat kidney. At higher concentrations, dehydroepiandrosterone sulfate and several carboxylic acids also stabilized pyridoxal phosphate binding. Similar stabilization was observed with partially purified serine transhydroxymethylase of rabbit liver and with the tyrosine-[alpha]-ketoglutarate transaminase of freshly homogenized rat liver.Tyrosine transaminase activity in fresh liver homogenates (1 gm liver per 12 mls of water) lost 50% of its activity during incubation for 1 hr at 37[deg]. This activity could not be restored by adding pyridoxal phosphate although the loss could be largely prevented by the presence of 10-5 M PLP and by 10-3 M [alpha]-ketoglutarate. Benzoate, p-aminobenzoate, [alpha]-naphtoate, salicylate, and diethylstilbestrol disulfate, all of which were shown previously to induce hepatic tyrosine transaminase , retarded the inactivation. Hydrocortisone or diethylstilbestrol disulfate injections also resulted in decreased rates of inactivation in liver homogenates prepared 3 hours after injection as compared with those prepared immediately after injection.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33454/1/0000857.pd