New substrate for determination of serum lecithin:cholesterol acyltransferase

Serum lecithin:cholesterol acyltransferase (LCAT) was estimated by enzymatically measuring the decrease in unesterified cholesterol after incubation of serum with liposomes. A high-performance liquid chromatography (HPLC) study showed the uptake of the lipids of liposomes by serum high density lipoprotein. Of all the examined liposomes prepared from cholesterol and various synthetic phosphatidylcholines, liposomes with dimyristoylphosphatidylcholine (DMPC) were found to be the most reactive in the LCAT reaction. When serum was used as an enzyme source, addition of purified apolipoprotein A-I, which is known to be an endogenous activator of LCAT, to the assay mixture resulted in a slight decrease in enzyme activity. Using DMPC-cholesterol liposomes as the substrate, the LCAT activities in 120 human sera showed a mean value of 485.4 +/- 64.6 nmol/hr per ml (mean +/- SD), which is 4.4- to 5.4-fold higher than the values obtained by self-substrate methods. LCAT activity was a linear function of the serum sample volume up to 670 nmol/hr per ml and coefficients of variation (CV) less than 4% were obtained under the standardized conditions. Moreover, when partially purified LCAT was added to various heat-inactivated sera, the activity was efficiently recovered. These results suggest that this method is sensitive, reproducible, and not greatly influenced by serum components.