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We have described the protein, enzyme, and isoenzyme characteristics of three gross fractions of both normal and abnormal human sera, as obtained by application of high centrifugal force. In the absence of solution gradient, such fractions represent pure and undiluted elements of the serum; they are reproducible and sufficiently voluminous for extensive analytical study. Specific features of these fractions may be of interest in planning additional or peripheral investigations of serum. Fraction I appears to be a fair sample of native serum water; Fraction II appears to segregate the pre-β-globulin lipoprotein component observed in some sera; Fraction III is a concentrated solution of LDH, CPK, and SGOT enzymes, and γ-globulin-rich protein, which is hpoprotein-free. More than 95% of the LDH, CPK, and SGOT is located in Fractions II and III. Quantitative activity and qualitative nature of these enzymes appears to survive the stress of the ultracentrifuge technique relatively well.

Ultracentrifugation at a density of 1.21 of mixtures of apoHDL and HDL in which one component was labeled with 125 I showed that interchange of labeled protein took place. Since no evidence of intermediates was obtained it seems more probable that these results are explained by interchange of the relatively small protein subunits (mol.wt. approx. 15,000) between apoHDL and the large HDL complex (mol.wt. approx. 200,000) than that lipids move from HDL to apoHDL, as has been previously suggested. The top fraction obtained by centrifuging a mixture of [ 125 I]apoHDL and HDL at d 1.21 lost less than 20% of the labeled protein when it was recentrifuged at the same density. The fraction of [ 125 I]apoHDL which floated at d 1.21 in the presence of HDL was larger than the fraction of [ 125 I]HDL which sedimented to the bottom in the presence of apoHDL, indicating the occurrence of some complex formation between HDL and [ 125 I]apoHDL in addition to the interchange of labeled protein subunits. Similar results were obtained on electrophoresis; [ 125 I]HDL after incubation with apoHDL gave zones corresponding to HDL and to apoHDL both of which contained 125 I. Complex formation between [ 125 I]apoHDL and LDL was demonstrated by ultracentrifugation of a mixture. A small fraction of the labeled protein floated at d 1.063 but on recentrifugation half of the labeled protein sedimented. When trace amounts of [ 125 I]apoHDL were mixed with serum and the lipoprotein fractions separated by ultracentrifugation, 60% of the label was recovered in the HDL fraction and only 1.5% in the LDL fraction.