Lipoprotein lipase-mediated sequestration of long-chain polyunsaturated triacylglycerols in serum LDL normal and hypothyroid rats

Rat serum VLDL, unlike human, contains significant proportions of triacylglycerols with polyunsaturated C₂₀ and C₂₂ fatty acids. Hypothyroidism in this species is characterized by low levels of serum VLDL, the accumulation of LDL, elevated levels of lipoprotein lipase and depressed hepatic lipase activity. The hypothyroid rat thus represents an interesting model in which to study hepatic VLDL metabolism and the substrate specificity of lipoprotein lipase. This report shows that serum IDL and LDL in both euthyroid and hypothyroid rats contain progressively enhanced proportions of triacylglycerols with polyunsaturated C₂₀ and C₂₂ fatty acids when compared to VLDL. Hypothyroidism resulted in a decrease in the proportion of 22:06 fatty acid within the serum VLDL triacylglycerols when compared to euthyroid VLDL. Lipolysis of VLDL from euthyroid rats in vitro using the perfused rat heart system resulted in increases or sequestration o0f triacylglycerols containing long-chain polyunsaturated fatty acids within the IDL fraction similar to those seen in vivo. It is concluded that lipoprotein lipase-mediated hydrolysis of VLDL triacylglycerols and the conversion of VLDL to IDL and LDL in the rat results in a progressive sequestration of the longer-chain polyunsaturated triacylglycerol molecular species with the IDL and LDL.