1. The serum lipoproteins, apoproteins and lecithin—cholesterol acyltransferase activity of three patients with abetalipoproteinaemia have been studied.

2. Concentration of lipoproteins with a density of less than 1·063 was found to be 5–6 mg/100 ml of plasma. The protein part consisted only of apoAI polypeptides.

3. Concentration of total serum high-density lipoproteins was 90–100 mg/100 ml. After total delipidization of this fraction, apoproteins were separated by column chromatography. All apoA and most of the apoC peptides could be demonstrated by immunochemical methods and polyacrylamide gel electrophoresis.

4. Quantification of individual polypeptides indicated that the proportions apoAI: apoAII: apoAIII in high-density lipoproteins were not different from those observed in normal subjects. ApoCIII1 was absent in the patients' serum. In the remaining apoC peptides differences in the distribution compared with normal subjects could be demonstrated.

5. With antibodies to lipoprotein B, acetylated lipoprotein B or apolipoprotein B no reaction could be observed with abetalipoproteinaemic serum or any density fraction tested including density 1·23 infranate.

6. Isoelectric focussing of prestained lipoproteins in complete patients' serum revealed the absence of four major lipoprotein families present in normal serum.

7. Lecithin—cholesterol acyltransferase activity of abetalipoproteinaemic serum was reduced to a value of about 50% of normal but lipoproteins of patients' serum could be utilized as a substrate only to a minor degree. The maximum activity was observed only if lipoproteins of normal serum were added to the assay system.

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