Phosvitin kinase from the liver of the rooster. Purification and partial characterization

Phosvitin kinase, the enzyme that phosphorylates the egg yolk protein phosvitin, has been purified more than 8000 fold from the supernatant of rooster liver, a tissue that synthesizes phosvitin only after estrogen administration. The purified enzyme prepared from the estrogen treated liver was recovered in high yield (49%) and was very stable at 0-4°. It sedimented in sucrose as a single peak with a s(20,w) of approximately 7.6. Preliminary studies indicated that the native enzyme consists of several subunits with the major component having a molecular weight between 40,000 and 50,000. Rooster liver phosvitin kinase phosphorylated phosvitin at a 10 fold higher rate than it phosphorylated casein. It did not phosphorylate histone or protamine. Mg²⁺ stimulated the rate of phosphorylation, while Co²⁺ and Mn²⁺ were inhibitory. Phosvitin kinase activity was independent of cyclic adenosine 3',5' monophosphate. A unique property of the enzyme was its ability to utilize GTP as well as ATP as a phosphate donor for phosphorylation of phosvitin. Despite the fact that estrogen administration to roosters led to a marked induction in the hepatic synthesis of phosvitin, there was no significant difference in the phosvitin kinases prepared from treated and untreated livers in either the specific activity or the apparent K(m) for ATP (7.7 μM). These data suggest that phosphorylation of phosvitin by phosvitin kinase is probably not a rate limiting event in the overall process by which estrogen induces specific protein synthesis in rooster liver.