Two physiological substrate-specific casein kinases are present in the bovine mammary gland.

Two species of casein kinase from lactating bovine mammary gland have been identified; a Ca2+- and CM-independent casein kinase and a Ca2+- and CM-dependent casein kinase. The Ca2+- and CM-independent casein kinase phosphorylates previously dephosphorylated alpha s1-, beta- or kappa-casein while the Ca2+- and CM-dependent casein kinase prefers previously dephosphorylated beta- or kappa-casein as substrates. Two activities are indicated by their substrate specificity, sensitivity to Ca2+ and CM, pH maxima, and differential solubilization by anionic detergents. The presence of a regulated casein kinase in the lactating mammary gland suggests that casein phosphorylation may be a regulator of micelle formation or secretion.