[Hydrolysis of prolactin by a serine proteinase from mammary gland secretory cell mitochondria].

Mitochondrial proteinase isolated from secretory cells of the mammary gland of lactating rats able to hydrolyze 125I-labeled and native prolactin (PRL) has been studied. The enzyme represents a serine proteinase and is localized in the inner mitochondrial membrane. The molecular mass of the enzyme is 17-18 kDa, pH optimum is at 8.0-9.0. Partial purification of the enzyme has been carried out. The Km constant for 125I-PRL is equal to 10(-6) M, that for 2% hemoglobin is 1.2 x 10(-4) M. Analysis of products of rat and ovine PRL hydrolysis by proteinase using high performance liquid chromatography and PAAG electrophoresis revealed the formation of large-size fragments of the hormone. A possible role of proteinase in the mechanism of PRL action on mammary gland tissues is discussed.