Peptidases in the Rat Hypothalamus Inactivating Thyrotrophin-Releasing Hormone (Trh)

Abstract

Peptidases capable of inactivating thyrotrophin-releasing hormone (TRH) have been demonstrated in the hypothalamus. With the development of a specific radioimmunoassay for TRH, this method was used to further study the enzymes acting on the releasing hormone. Whole hypothalamic homogenates from male and female rats inactivated TRH, with greater peptidase activity being found in the female animals. Separation of the homogenates into particulate (microsomal and mitochondrial) and supernatant (soluble/cytoplasmic) fractions showed approximately the same amounts of enzyme activity in both fractions, while dialysis of the fractions slightly reduced the TRH peptidase activity present, suggesting that a diffusible co-factor might be partially involved in the releasing hormone's degradation. These results confirm the presence of TRH-inactivating peptidases in the rat hypothalamus and suggest that the enzymes may be involved in some way in the mechanisms by which the brain controls thyrotrophin release by the anterior pituitary.

Further studies on the inactivation of thyrotrophin releasing hormone (TRH) by enzymes in the rat hypothalamus

“…The possibility that the peptidases inactivating TRH might require a co-factor has been inferred from the decrease in enzyme activity caused by dialysis (Griffiths et al 1975): this could well be a metal ion, which would normally be removed along with other small molecules likely to interfere with the enzymes' activity. From the effects of the va¬ rious metal ions added, manganese was the only one found to have a stimulatory effect, so this may be present at the enzymes' active sites.…”

“…For the hypo¬ thalamic regulatory hormones, there has been in¬ creasing interest in their inactivation by enzymes which are present in various tissues, and it is possible that these enzymes may contribute to the mechanisms regulating the hypothalamic hor¬ mones' release and duration of action (see review by Griffiths 8c Kelly 1979). Thyrotrophin-releasing hormone (TRH), the tripeptide pyroGly-His-Pro NH2, is rapidly degraded by enzymes in peripheral and pituitary-portal blood (Redding & Schally 1969;Bauer 1976;Knigge et al 1976;White et al 1976); other sites of inactivation include the liver and kidney, skeletal muscle, pituitary and several areas of the brain (Redding & Schally 1969;Mudge & Fellows 1973;Bassiri & Utiger 1974;Griffiths et al 1975Griffiths et al , 1978Visser et al 1976). …”

“…0.25 M sucrose in a Gallenkamp TKW-300-030T glass homogenizer at 4°C. The resulting homogenate was centrifuged at previously described (Griffiths et al 1975) to produce a particulate fraction containing microsomes and mitochondria and a superna¬ tant (soluble/cytoplasmic) fraction. After re-suspension of the particulate fraction in 0.8 ml double-distilled water, both fractions were dialysed in Visking dialysis tubing for approximately 20 h against double-distilled water at 4°C.…”

Communications

“…The brain was rapidly removed and tissue from the hypothalamus, thalamus, cortex, cerebellum and pituitary dissected out. Pooled tissue from ten rats (twenty rats for the pituitary) was homogenized in 025 m sucrose and centrifuged to yield a supernatant fraction and a particulate fraction (Griffiths, Hooper, Jeffcoate & White, 1975). Incubations were performed at 37 'C in a total volume of 1 ml.…”

Concentrations of Thyrotropin-Releasing Hormone (TRH) and Deamido-TRH, and TRH Deamidase Activity in Brain*

“…These data, while confirming the presence of a TRH deamidase in the brain, are compatible with the hypothesis that the percentage of TRH produced in the brain which undergoes deamidation in the brain in vivo is small. (Endocrinology 107: 443,1980) T RH IS enzymatically degraded by serum and a number of tissues, including the brain (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). When TRH is incubated with serum (3,6,8,11) and with brain homogenates (8,9,10,12), compounds which migrate in certain chromatographic systems with pGlu-His-Pro (TRH-OH), the deamidated analog of TRH, are formed.…”