Biopterin cofactor biosynthesis: independent regulation of GTP cyclohydrolase in adrenal medulla and cortex.

Guanosine triphosphate cyclohydrolase, the enzyme that is apparently rate-limiting in biopterin biosynthesis, is increased in adrenal cortex and medulla of rats treated with insulin or reserpine. Denervation and hypophysectomy block the increase in medullary and cortical enzyme activity, respectively, whereas cycloheximide presents the increase in both tissues. These results provide evidence for induction and regulation of guanosine triphosphate cyclohydrolase.