Intestinal lipoprotein metabolism.

The intestine has long been recognized as n organ active in lipoprotein formation. This is hardly surprising when one considers the large quantities of dietary and endogenous lipids that traverse the intestinal mucosa daily and are absorbed into the mesenteric lymph as lipoproteins. Until recently, the major focus of research on intestinal ipoprotein metabolism was concerned with the intestine as an absorptive organ, exploring triglyceride absorption and secretion during a variety of experimental and clinical situations. In recent years an additional focus has emerged, namely, that of the intestine as an important source of lipoprotein constituents for plasma lipoproteins. The rapid modifications which lipoproteins undergo after secretion have served to disguise the contribution of a given organ since plasma lipoproteins may differ markedly from precursor lipoproteins analyzed closer to the synthetic source. This is particularly true of intestinal lipoproteins such as chylomicrons which are rapidly metabolized, distributing constituents such as phospholipids and apoproteins to other plasma lipoproteins. It has therefore become increasingly important to define the composition and factors influencing the secretion of lipoproteins as close to the synthetic source as possible. In the case of the intestine, such analyses can best be carried out on intestinal mesenteric lymph lipoproteins or within the intestinal mucosa directly. It is not the intent of this review to provide a comprehensive review of plasma lipoproteins. Several reviews (1 -3) have appeared over the last few years summarizing the current state of knowledge of the composition, structure, distribution, and function of the lipoproteins and their apoproteins. Rather, this review will concentrate on the available evidence for the intestinal synthesis and secretion of lipoproteins with emphasis on lipoprotein apoproteins and the factors influencing their secretion. In addition, the peripheral metabolism of intestinal lipoproteins will be briefly reviewed to indicate mechanisms by which lipoproteins of intestinal origin participate in systemic lipoprotein metabolism.