TRH-degrading enzyme activity in peripheral and pituitary-portal plasma of the rat

The enzymatic degradation of TRH was measured in peripheral and pituitary-portal plasma. Enzyme activity in peripheral plasma was 3.20 pg/min/mug plasma protein; in pituitary-portal plasma the rate of degradation was only 0.83 pg/min/mug protein. After dialysis, which removed virtually all endogenous TRH and LHRH, enzyme activity in peripheral and portal plasma was 2.24 and 0.60 pg/min/mug respectively. These data suggest that portal plasma contains large concentrations of unidentified substances which are competitive inhibitors or substrates for the enzyme or that portal plasma contains less TRH-degrading enzyme.