DEVELOPMENT OF METHODS FOR ANALYZING PLASMA-LIPOPROTEIN CONCENTRATIONS AND ASSOCIATED ENZYME-ACTIVITIES AND THEIR USE TO MEASURE THE EFFECTS OF PREGNANCY AND LACTATION IN CATS

Methods available for measurement of plasma lipoprotein-cholesterol co ncentrations and activities of lipoprotein lipase, hepatic lipase, lec ithin:cholesterol acyl transferase (LCAT), and cholesteryl ester trans fer protein were adapted for use in cats. A combined ultracentrifugati on/precipitation procedure was used to isolate very low-density lipopr oteins (VLDL), then to separate low-density lipoproteins (LDL) from hi gh-density lipoproteins (HDL). The reagent used, 92 mM heparin-mangane se chloride, provided complete precipitation of LDL with only trace an d insignificant contamination by HDL. Efforts to selectively measure l ipoprotein lipase activity in plasma, collected after IV injection of heparin, by inhibiting hepatic lipase with sodium dodecyl sulfate were unsuccessful, and the activity of this enzyme was calculated as the d ifference between total and hepatic lipase activities. The latter was measured in the presence of high salt concentration to inhibit lipopro tein lipase. Cholesterol esterifying activity was identified in feline plasma and was typical of LCAT, in that it was dependent on apolipopr otein A-I as a cofactor. The intra-assay and interassay coefficients o f variation for measurement of lipoprotein lipase, hepatic lipase, and LCAT activities were 18.4, 4.6, and 7.2%, and 20.4, 10.7, and 5.3%, r espectively. Appreciable cholesteryl ester transfer protein activity w as not detected in either undiluted or diluted plasma. These methods w ere subsequently used to investigate the effects of pregnancy and lact ation on lipoprotein metabolism in a group of 10 queens. Plasma concen trations of cholesterol and triglycerides were unaltered during pregna ncy, but the concentrations of VLDL-cholesterol increased and those of HDL-cholesterol decreased. During lactation, the concentrations of ch olesterol and triglycerides decreased owing to reductions in VLDL-chol esterol and LDL-cholesterol concentrations and continued suppression o f HDL-cholesterol. These changes were associated with alterations in t he activities of lipoprotein lipase, which increased after parturition , and hepatic lipase, which increased during pregnancy and lactation, that may help explain their metabolic origins. The activity of LCAT re mained unchanged.