Abstract
Triglyceride moieties of circulating chylomicrons and very low density lipoproteins (VLDL) are natural substrates for lipoprotein lipase (LPL) (EC 3.1 .1.34), an enzyme functional at the vascular endothelial surface [ 1,2]. A synthetic substrate, triolein emulsified in glycerol [3], has gained widespread use. We had employed this latter substrate to compare heparinreleasable LPL with the remaining, non-releasable LPL in the perfused rat heart [4]. The 2 fractions (releasable and non-releasable LPL) displayed a major difference in their app. Km-values. We associated the high affinity nature of the ‘releasable’ fraction with the functional, endothelial-bound lipase. The low affinity nature of the ‘non-releasable’ fraction was compatible with its presumed role as precursor of the functional lipase and its suggested location in the parenchymal cells of the tissue [ 1,5,6]. Male Sprague-Dawley rats (Hilltop Lab), 200300 g, were fasted overnight. The preparation of the lipase fractions from perfusate and residual heart tissue has been described [4]. The natural substrate used was chyle, collected from the thoracic duct of rats pre-fed with 1-2 ml evaporated milk containing 100 PCi [ 1 -14C] palmitate (Rose Chemical Co.). More than 96% of the radioactivity was incorporated into the chylomicron fraction as revealed by isolation of the latter by centrifugation at 30 000 X g for 45 min at 16’C [ll]. The triglyceride content was determined enzymatically [ 121. Synthetic substrate was prepared by emulsifying triolein in glycerol with lecithin as detergent [3,4].
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