EFFECTS OF ALBUMIN AND APOLIPOPROTEIN C-II ON THE ACYL-CHAIN SPECIFICITY OF LIPOPROTEIN-LIPASE CATALYSIS

In this study we used monoacid triacylglycerols of various acyl-chain lengths as substrates for probing the active-site structure and substr ate specificity of lipoprotein lipase (LPL). An unexpected finding was that the albumin ligand binding site is accessible not only to long-c hain fatty acids for its recognized functional.role as a fatty acid ac ceptor, but also to short- and medium-chain monoacid triacylglycerol s ubstrates. The observed striking inhibitory effect (99%) of albumin on the LPL-catalyzed hydrolysis of trihexanoylglycerol is probably the r esult of the high affinity interaction of albumin with this substrate. Spectrophotometric analyses indicated that there is one high affinity binding site per albumin molecule (apparent K(D) - 1.8 +/- 0.9 mum) f or the interaction with trihexanoylglycerol. Despite LPL acyl-chain sp ecificity being obscured by the substrate binding effect of albumin, a systematic study of the lipolysis reaction under various assay condit ions demonstrated that tributyroylglycerol represents the best substra te for LPL, and the preferential order of LPL catalysis for both the b asal and apoC-II-activated activities is: C4>C6>C8> C10>C12> C18:1. In some assay conditions, the presence of albumin affects the above-ment ioned order, which can be attributed to substrate binding by albumin, rather than an alteration in the specificity of LPL. The synergistic e ffect of apoC-II and albumin resulted in the preferential activation o f LPL for the hydrolysis of long-chain triacylglycerols. Even with opt imal assay conditions for the hydrolysis of long chain triacylglycerol s, there is still a preferential reactivity of LPL with short- and med ium-chain triacylglycerols.