Abstract
Growth hormone (GH) and prolactin (PRL) share significant structural homology. We have previously characterized the phosphorylation of bovine PRL and wish to determine whether a similar kinase activity phosphorylates bovine GH. Phosphorylation of bovine GH was performed using [alpha-32P]ATP labeling of subcellular fractions. Bovine GH phosphorylation was dependent on Zn2+ or Cu2+ with apparent Km's of 0.9 and 1.0 mM, respectively, and a pH maxima of 7.0. The apparent Km's of bovine GH kinase activity for exogenous bovine GH and ATP were 30 microM and 376 microM, respectively. Exogenous bovine PRL served as a competitive substrate, increasing the apparent Km for bovine GH by threefold compared to the Km determined without exogenous bovine PRL. We conclude: 1) in vitro phosphorylation of bovine GH occurs under conditions that are consistent with those found in anterior pituitary cells, and 2) a similar kinase activity phosphorylates both bovine PRL and GH.
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