Endocrinology, Vol 124, 2818-2820, Copyright © 1989 by Endocrine Society

ARTICLES

Type I, 5'-monodeiodinase activity in the lactating mammary gland

C Aceves and C Valverde
Departamento de Fisiologia, Instituto de Investigaciones Biomedicas, Universidad Nacional Autonoma de Mexico.

Mammary homogenates from lactating, weaned, pregnant, and nonpregnant rats were analyzed for 5'-monodeiodinase activity (5'-MA). Only lactating glands exhibited significant enzyme activity. Competitive analysis showed that mammary 5'-MA is not inhibited by high (10 microM) concentrations of T4 or T3, but is highly sensitive to prophythiouracil (5 mM). Kinetic parameters demonstrate an acompetitive bisubstrate enzymatic mechanism known as the ping-pong type. The Km and maximum velocity for rT3 were 0.40 microM and 1.42 nmol/mg.min, respectively. These results provide the first evidence that 5'-MA is present in the lactating mammary gland and suggest that this enzymatic activity corresponds to type I enzyme.

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