The fatty acid and positional specificities of lipoprotein lipase.

Lipoprotein lipase has been identified in extracts of adipose tissue and heart and seems to be identical to the lipase that appears in plasma after the injection of a polyanion (cf. 1,2). The most unique characteristic of this lipase is its specificity for triglycerides in the form of a natural or synthetic lipoprotein complex. This paper describes investigations on the specificity of lipoprotein lipase with respect to the type of fatty acid and the position of the fatty acid (a! or p) in the triglyceride. Chylomicrons obtained from rats fed either olive oil, corn oil, cream, or cocoa butter were incubated with lipoprotein lipase. The free fatty acids produced by hydrolysis were isolated, and their molar percentage composition compared to that of the triglyceride substrate. If the enzyme catalyzed preferentially the hydrolysis of ester bonds involving certain specific fatty acids, one would expect those fatty acids to comprise a greater percentage of the free fatty acids than of the triglycerides. One possible difficulty in this approach is that a fatty acid specificity (or lack of specificity) might be concealed by a combination of a nonrandom distribution of fatty acids in the triglyceride substrate and an enzymic positional specificity. To control this, the chylomicrons were also degraded by pancreatic lipase. This enzyme is known not to have a fatty acid specificity (among the long chain fatty acids under consideration) and to cleave preferentially fatty acids esterified at the a position (3, 4). It serves, therefore, as an analytical tool for the determination of the distribution of fatty acids between the (Y and p positions of the chylomicron triglycerides (5, 6). Similar results were obtained when the chylomicrons derived from olive oil, corn oil, and cream were analyzed in this way. In each type of chylomicron the composition of the fatty acids at the cy position was not significantly different from the composition of the fatty acids which occupied the p position. However, the compositions of the fatty acids at the cx and fl positions were very different in the chylomicrons derived from cocoa butter. These chylomicrons, therefore, were very useful in determining the positional specificity of lipoprotein lipase. The relative rates of hydrolysis of the a: and /3 esters were also determined by comparing the rates of production of free fatty acids and formaldehydogenic compounds (after periodate oxidation). Lipoprotein lipase was found to have neither fatty acid nor positional specificity.