Biosynthesis of triglycerides in freshly secreted milk from goats

It has been confirmed that freshly secreted milk from goats contains triglyceride synthetase activity which is not stimulated by addition of the usual range of cofactors, e.g. coenzyme A, adenosine 5′‐triphosphate, Magnesium ions or α‐glycerophosphate. On the other hand, free glycerol did stimulate the reaction, although only low levels of glycerol‐kinase could be detected. The products were principally, triglycerides, diglycerides, and phosphatidic acid, i.e. those expected of the phosphatidic acid pathway of triglyceride biosynthesis together with some phosphatidyl choline and phosphatidyl ethanolamine. In addition, some of the enzymes of the monoglyceride pathway appeared to be present since 2‐O‐alkylglycerols, in particular, were converted to monoacyl‐monoalkyl glycerols together with trace amounts of diacyl alkyl‐glycerols. The milk incorporated a wide variety of isotopically labeled fatty acids into lipids at widely differing rates (18∶2 at 10 times the rate of 18∶0, for example) and stereospecific analysis confirmed that fatty acids entered all three positions of the sn‐triacyl glycerols according to quite specific patterns. Stearic acid was desaturated rapidly to oleic acid, and virtually all the 18∶1 formed was esterified (to all three positions in the triglycerides also). Freshly secreted milk is a potentially useful alternative to tissue obtained surgically as a source of mammary triglyceride biosynthetic enzymes for the study of milk fat biosynthesis in vitro.