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1. Human mesenteric lymph chylomicrons were isolated from chylous ascites fluid by ultracentrifugation and agarose/gel chromatography and their apoprotein composition was analysed by dodecylsulfate/polyacrylamide gel electrophoresis, analytical isoelectric focusing and immunochemically. Major components of mesenteric lymph chylomicrons were apoprotein A-I, proteins of Mr < 15000 including the C-group apoproteins and a protein of Mr 46000. Minor components were apoprotein E and a protein of Mr∼ 200000 (B-like protein). This apoprotein composition was qualitatively identical with that of chylomicrons from intestinal lymph of the rat, but was distinctly different from plasma chylomicrons of humans with fasting chylomicronaemia.

2. The protein of Mr∼ 46000 has been isolated by preparative dodecylsulfate/polyacrylamide gel electrophoresis from human and rat lymph chylomicrons and was compared to a protein of identical Mr present in rat high-density lipoproteins (apolipoprotein A-IV) and in the ϱ < 1.006 g/ml serum lipoprotein fraction of individual humans with alimentary hypertriglyceridaemia. In both species the 46000-Mr proteins isolated from lymph and serum were identical according to amino acid composition and isoelectric point in 6 M urea. The human proteins from both sources were also immunologically identical. The similarities in the molecular properties of the human apolipoprotein and rat apolipoprotein A-IV indicate that these proteins are homologous.

3. Plasma levels of human apolipoprotein A-IV determined by electroimmunodiffusion were 14.15 ± 3.66 mg/100 ml (n= 59), but > 90% of the protein was unassociated with the major lipoprotein fractions.

It is concluded, that apolipoprotein A-IV is a main protein component of human lymph chylomicrons, that is removed from the particles in the plasma compartment.