Lipid transport systems: some recent aspects in swine, cattle and trout during development

Lipids such as triacylglycerols, free and esterified cholesterol and phospholipids are essentially insoluble in water; lipoproteins, which are complex macromolecules of pseudomicellar structure, have evolved to ensure their efficient transport in the vascular system of vertebrates. Plasma lipoproteins are operationally classified according to their relative lipid and protein contents and thus according to density. The protein components, i.e. apolipoproteins, have a highly specialized structure contributing to particle stability and metabolism. The chemistry and structure of plasma lipoproteins and apoproteins is discussed and their biosynthesis, intravascular metabolism and cellular degradation briefly considered. Circulating concentrations of lipoproteins are under complex control, involving hormonal, nutritional and genetic factors; changes occurring in lipoprotein levels in pigs (Sus domesticus), cattle (Bos sp.) and rainbow trout (Salmo gairdnerii) during development are discussed. In pigs and calves, LDL predominate at the fetal stage and VLDL are absent. During suckling, this pattern is rapidly modified with HDL becoming the major class and low levels of VLDL appearing. These changes in part reflect high levels of exogenous triglyceride lipolysis consequent to the ingestion of fat-rich, maternal milk. With further growth, HDL predominate in both adult pigs and steers, although the LDL:HDL ratio is considerably lower in the latter (0.5-0.8 and approximately 0.2, respectively). Our studies in pigs suggest that the intestinal secretion of lipoproteins commences rapidly after birth since proteins akin to human apo-B48 and apo-B100 are detectable in plasma VLDL some 2-3 h after parturition. Although the trout is an oviparous vertebrate, LDL is also preponderate at the juvenile stage. With sexual maturity, LDL and VLDL levels diminish progressively, plasma HDL attaining concentrations as high as 1,500 mg/dl in adults. Our knowledge of the biochemical mechanisms at the origin of these developmental changes in lipid transport in both mammals and fish remains largely incomplete.