Abstract
A preparation of cerebral microvessels was used to demonstrate the presence of lipoprotein lipase and acid lipase activity in the microvasculature of rabbit brain. Microvessels, consisting predominantly of capillaries, small arterioles, and venules, were islated from rabbit brain. Homogenates were assayed for lipolytic activity using a glycerol-stabilized trioleoylglycerol-phospholipid emulsion as substrate. Lipoprotein lipase activity was characterized with this substrate by previously established criteria including an alkaline pH optimum, increased activity in the presence of heparin and heat-inactivated plasma, and reduced activity in the presence of NaCl and protamine sulfate. A different substrate, containing trioleoylglycerol incorporated into phospholipid vesicles, was used to reveal acid lipase activity that was not affected by heparin, plasma, NaCl, or protamine sulfate. Lipoprotein lipase did not show activity with the vesicle preparation as substrate. Intact microvessels, when incubated in the presence of heparin, release lipoprotein lipase into the incubation solution. In contrast, release of acid lipase activity from intact microvessels was not dependent on heparin. The data show the presence of both lipoprotein lipase and acid lipase in brain microvessels and suggest that lipoproteins are metabolized within the cerebral vasculature.
Highlights
A preparation of cerebral microvessels was used to demonstrate the presence of lipoprotein lipase and acid lipase activity in the microvasculature of rabbit brain
The I4C-labeledtriolein contained in the vesicle preparation was included within the column and eluted at a position consistent with that of unilamellar phospholipid vesicles.The emulsion preparation was shown previously to be suitable for the assay of lipoprotein lipase [18] and triolein-containing phospholipid vesicles were shown to be an effective substrate for an aortic acid lipase [19]
The lipolytic activity observed at acid pH and using the emulsion as substrate was not affected by heat inactivated plasma, as was shown in Fig. 2, and in separate experiments the effects of heparin, 1 M NaCl, protamine sulfate, and albumin were shown to have little or no influence on triglyceride hydrolysis when using the emulsion as substrate and performing the assay at pH 6.0
Summary
A preparation of cerebral microvessels was used to demonstrate the presence of lipoprotein lipase and acid lipase activity in the microvasculature of rabbit brain. Brecher and Kuan Lipase activity in rabbit brain microvessels 465 contained 75 p1 of the vesicle preparation and 50- 100 p1of enzyme solution in 0.15 M sodium acetate buffer, pH 4.0.
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