“In Vitro” Utilization of Labelled Esterified Fatty Acids and Glyceride Glycerol from Triglyceride-Rich Lipoproteins in Rat Adipose Tissue

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Abstract
Triglyceride-rich lipoproteins (chylomicrons and very low density lipoproteins) were labeled in vivo by injecting (U[uniformly labeled]-14C)-glycerol and (9-10(n)-3H)-palmitate in female rats. After purification, these lipoproteins contained most of the 3H in esterified fatty acids and the 14C in glyceride glycerol of neutral lipids. This preparation was incubated in vitro in the presence of either isolated adipocytes or epididymal fat pad pieces from male rats. With the incubation, a certain proportion of both 3H-esterified fatty acids and 14C-glyceride glycerol disappeared from the medium, the effect being greater when the incubations were performed with adipocytes than with fat pad pieces. Much greater radioactivity appeared in the lipids of adipocytes than in those of fat pad pieces at the end of 60 or 120 min incubation, and the incorporation of 3H being relatively greater than that of 14C. With the latter isotope, the label appeared not only in the glyceride glycerol fraction but also in the free and esterified fatty acids. Although it is known that lipoprotein lipase activity is lower in adipocytes than in fat pad pieces, the results indicate that, in the former preparation, the enzyme may be more accessible for the substrate. Glycerol released by the hydrolysis of lipoprotein glycerides may be partially incorporated into lipids by adipose tissue.