Protein kinase activity and protein kinase inhibitor in mouse kidney: effect of the X-linked Hyp mutation and vitamin D status.

Abstract

cAMP-dependent protein kinase, Ca+2- and phospholipid-dependent protein kinase, and protein kinase inhibitor activity were examined in renal homogenates and 20,000 X g supernatant fractions of normal and Hyp mice. In both genotypes, 70% of total renal cAMP-dependent protein kinase activity was recovered in the soluble fraction in which the activity ratio (without cAMP to with cAMP) of the enzyme was 0.35. The requirement for cAMP was not different for protein kinase of normal and mutant littermates, with an apparent Km for cAMP of 0.05 microM in both genotypes. Furthermore, vitamin D and calcium deficiencies did not significantly affect cAMP-dependent protein kinase activity in normal and Hyp mouse kidney. The concentration of the heat-stable protein kinase inhibitor protein in the 20,000 X g supernatant fraction was identical in normal and Hyp kidney. Whereas protein kinase inhibitor levels were increased 1.8-fold by vitamin D and calcium deficiencies in normal mice (P less than 0.001), no such increase was detectable in Hyp mice. Ca+2- and phospholipid-dependent-protein kinase (protein kinase C) activity in the 20,000 X g supernatant fraction comprised 50% of the total activity of kidney homogenates of both normal and mutant mice. The initial rate of protein kinase C was increased 1.5-fold in kidney supernatants of Hyp mice (P less than 0.001). In contrast, protein kinase C was not significantly different from normal in supernatant fractions of heart, spleen, and liver prepared from Hyp mice. The present demonstration of abnormally high renal protein kinase C activity in Hyp mice may serve to explain the relationship between the previously reported renal defects in brush border membrane phosphate transport and vitamin D metabolism in the mutant strain and elucidate the nature of the primary defect in the Hyp mouse.