Biochimica et Biophysica Acta, 420 (1976) 342-349 © Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands BBA 37248 ISOLATION AND CHARACTERIZATION OF THE MAJOR APOLIPO- PROTEIN FROM CHICKEN HIGH DENSITY LIPOPROTEINS RICHARD L. JACKSON, HU-Y. U. LIN, LAWRENCE CHAN and ANTHONY R. MEANS Departments of Medicine and Cell Biology, Baylor College of Medicine and The Methodist Hospital, Houston, Texas, 77025 (U.S.A.) (Received June 9th, 1975) (Revised manuscript received October 13th, 1975) SUMMARY High density lipoproteins were isolated from plasma of white Leghorn hens by ultracentrifugal flotation between densities 1.063 and 1.210 g/ml. After delipidation, the lipid-free proteins were fractionated by chromatography on Sephadex G-150 in urea; one major apolipoprotein was isolated and characterized. From its chemical, physical and immunochemical properties, the major apoprotein from hen high- density lipoproteins has characteristics similar to the major apoprotein of human high density lipoproteins, apoA-I. Thus the hen protein has been designated hen apoA-I. Hen apoA-I has a molecular weight of approximately 28 000 as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Its calculated molec- ular weight from its 234 constituent amino acids is 26 674. Hen apoA-I differed from its human counterpart by containing isoleucine. Treatment of hen apoA-I with carboxypeptidase A yielded a COOH-terminal sequence of Leu-Val-Ala-Gln. Auto- matic Edman degradation of the apoprotein gave an NHz-terminal sequence of Asp- GIu-Pro-Gln-Pro-Glu-Leu. Hen apoA-I had a circular dichroic spectrum typical of a-helical structures; the calculated helicity was 90 ~. Goat antisera prepared to hen apoA-I formed precipitin lines of complete identity to the hen apoprotein but lines of only partial identity to human apoA-I. These studies show that the major apoprotein from hen and human high-density lipoproteins have similar properties to each other suggesting a common physiologic function. INTRODUCTION In recent years, there has been considerable interest in the chicken as a model for the study of lipoprotein synthesis [1 ]. This model has been particularly useful since the administration of estrogen to cockerels and non-laying hens dramatically increases the synthesis of very low density lipoproteins [2-5]. As a prerequisite to studies on the effects of hormones on high-density lipoprotein synthesis, we now describe the iso- lation and characterization of the major apoprotein from hen high-density lipo- proteins. Since the results of these studies show that the major high-density lipoprotein protein from the hen has many of the same physical, chemical and immunochemical