The removal and metabolism of chylomicrons by adipose tissue in vitro.

Alimentary triglycerides are transported in the blood primarily in the form of emulsified particles called chylomicrons, which consist predominantly of triglycerides, but also contain a small proportion of cholesterol, phospholipid, and protein (1). Recently, Rodbell and Fredrickson (2) have found that one of the proteins associated with chylomicrons appears to be the same as the protein isolated from the plasma alpha or high density lipoprotein. This finding, plus the observation by Korn (3) that alpha lipoprotein “activates” or increases the rate of hydrolysis of triglycerides by lipoprotein lipase has enhanced the possibility that the metabolism of chylomicrons is mediated through the action of lipoprotein lipase as initially proposed by Korn (3). Although there has been some evidence for hydrolysis of chylomicron triglycerides intravascularly by lipoprotein lipase (4), the recent findings by Bragdon and Gordon (5) indicate that triglyceride removal from blood, at least in the rat, is independent of any significant hydrolysis of triglycerides during their brief circulating time. Accordingly, if hydrolysis of the chylomicrons is necessary for their removal from blood, it would appear more likely that the enzyme is acting in the tissues responsible for their removal. One very active site for triglyceride metabolism is adipose tissue which has been found to take up triglycerides selectively in vitro (7) and is very rich in lipoprotein lipase (8) (for a comprehensive review of chylomicron metabolism, see Fredrickson and Gordon (6)). The present investigation was undertaken to determine the role, if any, that chylomicron proteins and lipoprotein lipase play in the removal and metabolism of triglycerides by adipose tissue in a system in vitro. Evidence will be presented that triglycerides in the form of artificial emulsions can be removed intact and metabolized by adipose tissue as efficiently as chylomicrons. Lipolysis appears to be obligatory for the metabolism of triglycerides but not for their removal by adipose tissue.