GH kinase activity in bovine anterior pituitary subcellular fractions

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