Cholesterol ester hydrolase activity in mammary tissue of the lactating rat

Cholesterol ester hydrolase activity has been studied in mammary glands of rats. Subcellular fractionation of the glands obtained in mid‐lactation indicated that around 80% of the recovered activity was associated with particulate fractions. Two distinct cholesterol ester hydrolase activities were identified, one with an optimum pH of 7.5–9.0 and the second (approximately 5% of the total activity) with a more acidic pH optimum. Although the neutral cholesterol ester hydrolase had some properties in common with the lipoprotein lipase in mammary tissue, it was shown to be a separate entity by several criteria. Its activity could be increased following treatment with Mg‐ATP and cAMP‐dependent protein kinase, suggesting identity with the hormone sensitive lipase of adipose tissue. The cholesterol ester hydrolase activity in mammary glands just after parturition was greater than in glands obtained either from late‐pregnant or midlactating animals. The subcellular distribution of the neutral cholesterol ester hydrolase suggested that it may have a different function to the neutral cholesterol ester hydrolase of adrenals and other tissues. Nevertheless the fact that the activity of the enzyme can be modulated by cAMP‐dependent protein kinase suggests the possibility that hormonal control of this enzyme may be involved in the regulation of cholesterol metabolism in the mammary gland.