Comparative Studies of the Lecithin:Cholesterol Acyltransferase Reaction in the Plasma of Reptiles and Amphibians

Abstract

Cholesterol esterification in the plasma of reptiles and amphibians occurs, like that of mammals, through the enzyme, lecithin:cholesterol acyltransferase (LCAT). The initial rate of LCAT activity was measured in plasma from several different species of these animals and, whilst considerable inter- and intra-species variations were apparent, in general the activity in lizards was quantitatively comparable with that of mammalian species, whereas in snakes, amphisbaenids, and amphibians the activity was often much lower. In lizards and toads plasma LCAT activity was positively correlated with plasma triglyceride concentration. In lizards during prolonged fasting, LCAT activity and lipid levels were reduced in plasma, and the plasma lipoproteins from fasted animals had a reduced proportion of lecithin to unesterifled cholesterol and were poor substrates for LCAT in plasma from non fasted animals. The pH optimum for LCAT activity was close to 7 in all species studied, and activity was inhibited by reagents known to inhibit mammalian LCAT. The LCAT reaction in plasma from different reptiles and amphibians formed predominantly mono- or di- or poly-unsaturated cholesteryl esters (CE), depending upon species. LCAT from closely related species formed similar proportions of the different CE sub-classes, but other studies in which LCAT from one species was incubated with lipoproteins from a non-related species suggest that the lecithin composition of the substrate lipoproteins is an important determinant of the proportions of the different CE formed during in vitro incubation.

Key Words:

• amphisbaenids
• cholesterol
• cholesteryl esters
• lecithin-cholesterol acyltransferase
• lizards
• phospholipids
• snakes
• triglycerides