ACTIVATION AND INHIBITION OF LIPOPROTEIN-LIPASE IN MIXED MONOLAYERS OF MEDIUM OR LONG-CHAIN TRIGLYCERIDES AND PHOSPHOLIPIDS

We evaluated the activation and inhibition effects of phosphatidylchol ine (PC) and sphingomyelin (SM) on lipoprotein lipase (LPL) for medium or long chain-triglycerides (TG) in monolayers at the air/water inter face. Monolayers of medium chain-TG, being in an expanded state at a s urface pressure of 15 mN/m, showed low susceptibility to LPL in the su bphase. Adding 50 mole% of PC or SM into these monolayers reduced the partial molecular area of the TG and enhanced the LPL activity. Monola yers of long chain-TG, being in a condensed state, showed high suscept ibility of LPL either with or without PC. SM added to the long chain-T G monolayers, however, inhibited the LPL action. We investigated the i nteraction between TG and phospholipid on the basis of the collapse pr essure-measurements of mixed monolayers. For medium chain-TG monolayer s, PC and SM gave similar collapse pressure-composition profiles. Cont rary to this, SM gave a markedly higher collapse pressure of long chai n-TG than PC: SM stabilized the monolayer state of long chain-TG. Thes e results suggested that I) orientation of the acyl chains of TG molec ule in a monolayer is crucial for the LPL activity, and that II) stron g interaction between SM and long chain-TG retards the substrate-trans fer from the mixed monolayer to the catalytic pocket of LPL.