Studies on the Inactivation of Thyrotropin-Releasing Hormone (TRH)

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Abstract
Inactivation of porcine thyrotropin-releasing hormone (TRH) by plasma fractions in vitro was determined using mice maintained on a low iodine diet and pre-treated with 5 μCi of 125I and 0.085 μg of triiodothyronine. Incubation of TRH with porcine, bovine, or human serum caused a complete inactivation in 30 min. The optimum pH for the inactivation was about 7 and the optimal temperature was between 30 and 40°. The rate of inactivation of TRH was proportional to the enzyme concentration and time. Preheating rat plasma to 56° for 30 min greatly reduced this inactivation. When plasma fractions of porcine, bovine, and human origin were incubated with TRH in Krebs-Ringer bicarbonate, pH 7.4, at 37° for 30 min, alpha, beta, and gamma globulin fractions caused an 80-90% inactivation of added TRH. Albumin and fibrinogen caused a 40-50% reduction in TRH activity while the beta-lipoprotein fraction only induced a slight inactivation of TRH. Incubation of TRH with slices of rat liver, kidney, brain cortex, and skeletal muscle tissue also abolished TRF activity. Prior boiling of these tissues reduced the inactivation of TRH. These results are best explained by the presence of an enzyme in the plasma and various tissues which is capable of inactivating TRH.

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