Effect of Cortisone and Thyroxine on Aromatic Amino Acid Decarboxylation

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Abstract
The decarboxylation of o-tyrosine and 5-hydroxytryptophan has been studied in tissue homogenates of different organs of both normal and pyridoxinedeficient rats. In pyridoxine-supplemented rats, the apodecarboxylase of liver and brain was more saturated with coenzyme than the kidney apoenzyme. When animals were fed a pyridoxinedeficient diet, the liver and kidney apodecarboxylase was markedly reduced, although that of the brain remained intact. The decreased decarboxylating capacity of tissues from pyridoxine-deficient animals was attributable to depletion of the coenzyme and also to loss of the protein moieties of the enzymes in liver and kidney. The degree of saturation of apoenzyme with coenzyme was greater with 5-hydroxytrypotophan than with 0-tyrosine as substrate in tissues from both normal and pyridoxine-depleted animals. Cortisone administration increased the inherent hepatic decarboxylating activity for both substrates in normal and deficient animals. However, when these activities were measured in the presence of excess pyridoxal phosphate, the hepatic apoenzyme level was increased in pyridoxine-supplemented animals but remained unchanged in pyridoxinedeficient animals. The results indicate that cortisone administration increased the degree of saturation of apoenzyme with coenzyme in pyridoxine-deficient animals. Thyroxine administration suppressed the inherent hepatic decarboxylating activity for both substrates in normal and pyridoxinedeficient animals. The data indicate that thyroxine reduced apoenzyme formation or activity in both pyridoxine-supplemented and pyridoxine-deficient animals and also decreased the degree of saturation of apoenzyme with coenzyme in deficient animals.