Guinea pig apolipoprotein C-II: expression in E. coli, functional studies of recombinant wild-type and mutated variants, and distribution on plasma lipoproteins.

Guinea pig apolipoprotein C-II (apoC-II) lacks four amino acid residues in the amino-terminal, lipid-binding part compared to apoC-II from other mammalian species (Andersson et al. 1991. J. Biol. Chem. 266: 4074-4080). To explore whether this structural difference explains the low ability of guinea pig plasma to activate lipoprotein lipase in vitro, we have expressed guinea pig apoC-II in Escherichia coli and have constructed an insertion mutant with the four missing amino acid residues compared to human apoC-II. With a synthetic emulsion of long-chain triacylglycerols, both the wild-type guinea pig apoC-II and the insertion mutant stimulated lipoprotein lipase similar to human apoC-II, but with chylomicrons from an apoC-II-deficient patient, 5- to 10-fold more of both wild-type guinea pig apoC-II and the insertion mutant were needed. Studies of tryptophane fluorescence indicated a slight difference in how guinea pig apoC-II interacted with liposomes, and presumably with lipoproteins, as compared to human apoC-II. The level of apoC-II (11.5 +/- 5.4 microg/ml) was lower in guinea pig compared to human plasma, and most of guinea pig apoC-II was on HDL-like particles. These had decreased ability to donate apoC-II to lipid emulsions compared to human HDL. Some guinea pig apoC-II was associated with LDL which, as demonstrated by surface plasmon resonance, had higher affinity for lipoprotein lipase than human LDL, and inhibited rather than stimulated the lipase reaction in vitro. We conclude that while guinea pig apoC-II is fully competent to stimulate lipoprotein lipase, the sum of several different factors explains the low ability of guinea pig plasma to accomplish stimulation.