BABOON LECITHIN-CHOLESTEROL ACYLTRANSFERASE (LCAT) - CDNA SEQUENCES OF 2 ALLELES, EVOLUTION, AND GENE-EXPRESSION

Lecithin cholesterol acyltransferase (LCAT) is a key enzyme of cholest erol metabolism that catalyzes esterification of cholesterol for packa ging in high-density lipoprotein (HDL) particles. In this study, we cl oned and sequenced LCAT cDNA from baboon, a nonhuman primate model of atherosclerosis. LCAT sequences have been highly conserved over approx imately 25 million years since the divergence of the baboon and human lineages. The baboon and human sequences are 97% identical at the nucl eotide (nt) level and 98% identical at the amino acid (aa) level. Only 18% of the nt substitutions change the aa sequence (nonsynonymous sub stitutions). The aa substitutions between baboon and human LCAT do not alter key functional sites including the interfacial substrate active site, asparagine-linked glycosylation sites, or sites at which rare m utations cause human familial LCAT deficiencies. We also sequenced LCA T cDNA for a less common allele that is associated with higher LCAT ac tivities and altered lipoprotein phenotypes. There were no sequence di fferences between the two alleles, which suggests that genotypic effec ts are most likely due to allelic differences in gene expression. The tissue specificity of LCAT expression was investigated using an RNase protection assay calibrated with known amounts of synthetic human LCAT RNA. In a survey of baboon tissues, the highest levels of LCAT mRNA w ere found in the cerebellum and liver and trace amounts in the ileum, spleen and cerebral cortex.