Precursor-Product Relationship between Pools of Very Low Density Lipoprotein Triglyceride

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Abstract
The process of removal of triglyceride from the plasma may involve a sequential conversion of larger to smaller glyceride-rich lipoproteins. This has been studied within the species of lipoproteins comprising the very low density lipoproteins (VLDL) which transport the bulk of endogenously formed triglyceride. Palmitic acid-14C which was used to label the plasma glycerides was administered either as a prolonged constant infusion or as a pulse label. The specific activity-time curves of triglyceride fatty acids (TGFA) were analyzed both in total VLDL and in two subfractions of VLDL. The nature of the curves for total VLDL that were observed during the constant infusions were consistent with slow isotopic equilibration of precursors of VLDL-TGFA or with the presence of a precursor-product relationship between different components of VLDL-TGFA. The curves did not indicate any detectable differences in (fractional) turnover rates of independently metabolized pools of VLDL-TGFA. Differences in the specific activity-time curves of TGFA in two subfractions of VLDL (Sf > 100 and Sf 20-100) were consistent with a precursor-product relationship between TGFA in the two subfractions; again there was no indication of significant differences in (fractional) turnover rates. The specific activity-time curves of TGFA in the two subfractions of VLDL that were obtained with single injections of radio-palmitate showed a consistent difference in the rates at which TGFA became labeled in the two subfractions, being slower in the Sf 20-100 fraction. The findings from all experiments when considered together, were compatible with a precursor-product relationship that suggested that larger VLDL were converted to progressively smaller species as triglyceride was being removed.