A number of immunological methods have been employed to show that the small subunit of HL-A antigens, isolated either after papain treatment or after solubilization with detergent, is identical to beta(2)-microglobulin, a protein previously isolated from human urine and shown to be homologous in structure to constant region domains of immunoglobulins. Moreover, quantitative data indicate virtually total identity between the small subunit of HL-A antigens and beta(2)-microglobulin. Studies of the turnover of labeled HL-Aantigens from the lymphocyte surface indicate that the two subunits turn over at similar rates, although only the small subunit could be detected in the culture medium. The significance of these observations is discussed.

HL-A antigen solubilized with the non-ionic detergent, Brij 99, has been purified to about 50% of homogeneity from a cultured human lymphoblast line. It consists of two nonidentical subunits of 44,000 and 12,000 molecular weight (MW). Upon papain proteolysis the 44,000 MW peptide is converted by at least two cleavages to a 34,000 MW peptide, but the 12,000 MW peptide appears to be unchanged. Concomitantly, the apparent molecular weight in gel filtration chromatography under nondenaturing conditions in the presence of Brij 99 is reduced from 460,000 to 45,000. HL-A molecules produced by direct papain proteolysis of membranes and bypapain treatment of purified detergent-soluble HL-A are identical.