Evidence for deficiency of high density lipoprotein lecithin: cholesterol acyltransferase activity (alpha-LCAT) in fish eye disease

Familial LCAT deficiency and fish-eye disease

Familial LCAT deficiency is due to deficiency of plasma lecithin-cholesterol acyltransferase. The plasma is rich in free cholesterol and lecithin while cholesterol ester and lysolecithin levels are reduced. Analysis of the abnormal lipoproteins has helped our understanding of plasma lipid and lipoprotein metabolism in normals and in patients with liver disease. Proteinuria and anaemia are common and there is marked corneal lipid deposition. Eventually renal function deteriorates and dialysis and/or renal transplantation may be necessary. The human LCAT gene has been sequenced and been shown to be present on chromosomal segment 16q22-the region predicted on the basis of recombination studies as the site of the LCAT deficiency gene. The gene defect has been identified in some cases, but the mechanism remains unclear as the mutations were not in the region presumed to be the enzyme's active site. Only three cases of fish-eye disease have been described; all were elderly and had obvious corneal opacities. They had fasting hypertriglyceridaemia and increased VLDL. IDL and LDL were increased and were triglyceride rich. HDL, reduced by 90%, was mainly HDL3--with a high free and low ester cholesterol. LCAT activity in fish-eye plasma was normal but when measured in an exogenous substrate it was only 10-15% of normal. Fish-eye HDL is a substrate for purified LCAT, but fish-eye LCAT does not esterify free cholesterol of HDL (normal or fish-eye), although it esterifies free cholesterol of VLDL and LDL. It has been suggested that one type of LCAT activity acts on HDL (alpha-LCAT) and another on VLDL and LDL (beta-LCAT)--and that fish-eye disease is due to alpha-LCAT deficiency, and classical familial LCAT deficiency due to lack of both components.

The isolation and characterisation of a cDNA clone for human lecithin:cholesterol acyl transferase and its use to analyse the genes in patients with LCAT deficiency and fish eye disease

We have isolated cDNA clones coding for human lecithin:cholesterol acyl transferase (LCAT) from a liver-specific cDNA library by the use of two oligonucleotide probes based on the protein sequence. The clones span the sequence coding for the entire secreted LCAT, the 3' untranslated sequence and 12 amino acids of the signal peptide. The peptide sequence contains the conserved active site of serine lipases within a hydrophobic domain, flanked by a possible amphipatic alpha-helix. Only one gene for LCAT could be detected in genomic blots. We have used the cDNA as a probe to analyse the LCAT gene in patients suffering from LCAT deficiency and fish eye disease. No rearrangements or abnormal gene fragments were detected in these patients.

Purification of human plasma lecithin:cholesterol acyltransferase by covalent chromatography

Human plasma lecithin:cholesterol acyltransferase (LCAT, EC 2.3.1.43) has been purified more than 20,000 fold from plasma in 10% yield. This new procedure is composed of only four steps, including ultracentrifugation of plasma to yield a 1.21-1.25 kg/l density fraction, covalent binding of LCAT in this fraction to thiopropyl-Sepharose followed by adsorption of the enzyme to wheat-germ lectin-Sepharose for elimination of albumin and finally batch-wise treatment of the desorbed LCAT with hydroxyapatite to remove residual impurities. The purified enzyme was free of apolipoprotein A-I, A-II, B, C-I, C-II, C-III and E as checked by double immunodiffusion and SDS-electrophoresis, which latter method also demonstrated the absence of hitherto characterized lipid transfer proteins. Only traces of apolipoprotein D were present in the preparation as detected by immunoblotting. The purified enzyme retained alpha- and beta-LCAT activities. Non-denaturing and denaturing polyacrylamide gel electrophoresis yielded apparent molecular masses of 69 and 66 kDa, respectively, for the enzyme which on isoelectric focusing produced one major and one minor isoform with pI values of 4.20 and 4.25, respectively. Apolipoprotein A-I was required to transform artificial lecithin-cholesterol liposomes into substrates for the purified LCAT.

Net lipid transfer between lipoproteins in fish-eye disease plasma supplemented with normal high density lipoproteins

Native fish-eye disease plasma, which is deficient of both high density lipoproteins (HDL) and lecithin-cholesterol acyltransferase activity (alpha-LCAT), processing the free cholesterol of these lipoproteins, has been supplemented with normal isolated HDL2 or HDL3 and incubated in vitro at 37 C. After incubation for 0, 7.5 and 24 hr the very low density (VLDL) and low density (LDL) lipoproteins as well as HDL were isolated, and their contents of triglycerides, phospholipids and free, esterified and total cholesterol were quantified. The resulting net mass transfer of the different lipids revealed a functioning transfer of cholesteryl esters and all other analyzed lipids between the lipoproteins, although no de novo esterification of the HDL cholesterol by LCAT in this plasma occurred. In accordance with previous findings there was a functioning esterification process of the free cholesterol of the combined VLDL and LDL of fish-eye disease plasma. The present reports make it reasonable to conclude that the lack of HDL cholesterol esterification in this disease is not a result of a deficiency of cholesteryl ester transfer or lipid transfer activities.

Bile acids and plasma high density lipoproteins: biliary lipid metabolism in fish eye disease

Cholesterol in plasma high density lipoproteins (HDL) has been proposed to serve as preferential precursor for bile acid biosynthesis in the liver. Furthermore, a negative relationship between plasma levels of HDL cholesterol and biliary saturation with cholesterol has been reported in healthy females. We have performed metabolic studies on a female patient with fish eye disease, a familial condition where plasma HDL levels are reduced by 90% and the concentration of plasma triglycerides is moderately increased. Both the total production of bile acids and the net steroid 'balance' (reflecting total body cholesterol synthesis) were within the range seen in normolipidaemic as well as in hypertriglyceridaemic females. Also the biliary lipid composition and cholesterol saturation of bile were normal. A qualitative abnormality in the bile acid pattern was observed, however, in that the ratio between the synthesis of cholic acid and chenodeoxycholic acid was reduced. It is concluded that a low HDL cholesterol level is not necessarily associated with quantitative abnormalities of biliary lipid metabolism. The abnormal bile acid synthesis ratio may reflect changes in the hepatic precursor pools of cholesterol as the consequence of HDL deficiency, however.