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PROTEIN‐CARBOXYL METHYLTRANSFERASE OF THE BOVINE POSTERIOR PITUITARY GLAND: NEUROPHYSIN AS A POTENTIAL ENDOGENOUS SUBSTRATE
Author(s) -
Edgar D. H.,
Hope D. B.
Publication year - 1976
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1976.tb05160.x
Subject(s) - neurophysins , posterior pituitary , pituitary gland , methylation , biochemistry , endogeny , substrate (aquarium) , methyltransferase , vasopressin , chemistry , biology , medicine , endocrinology , hormone , ecology , gene
— The kinetics of methylation of neurophysin by the enzyme protein‐carboxyl methyltransferase (EC 2.1.1.24) was investigated, together with the subcellular distribution of enzyme and endogenous substrate in the bovine posterior pituitary gland. Using a preparation of the methyltransferase, purified from acetone‐dried powders of bovine posterior pituitary gland, bovine neurophysin was found to be a better substrate than those proteins usually employed, neurophysin having a lower apparent K m and higher V max than the other substrates. Arginine‐vasopressin did not inhibit the methylation of neurophysin, supporting the hypothesis that methylation and hormone‐binding sites are different. Vasopressin, however, increased the rate of methylation of neurophysin I by the methyltransferase. but was without effect on the rates of methylation of the other components of bovine neurophysin. After subcellular fractionation of fresh bovine posterior pituitary glands, some 77% of the recovered methyltransferase activity was found in the high‐speed supernatant fraction, being distributed throughout all the fractions similarly to lactate dehydrogenase. Neurophysin is therefore unlikely to become a substrate until released from the neurosecretory granules during stimulation of the gland. Some evidence is presented for the presence of an endogenous substrate other than neurophysin, in the high‐speed supernatant fraction.