A simple one-step hemolytic assay of the first component of human complement (C1) with polyethylene glycol treated human serum: details of the assay procedures and its application to the C1 activation study.

C1 was removed from human serum by polyethylene glycol (PEG) precipitation and the supernatant (C1 deficient serum, C1D) was used for assay of C1 hemolytic activity (C1D method). Serum concentration of PEG 6,000 ranging from 3.5-4% was determined to be suitable for preparation of C1D, but C1D prepared by PEG 4,000 were proved unsatisfactory for use. Oxidation of C1D by iodine treatment increased C1 activity by two-fold. The C1D method was comparable in sensitivity to the conventional method, and the correlation between the 2 methods was good (r = 0.94). The C1D method was shown to be a useful tool for the study of C1 activation, since the method specifically measured C1, but not C1 activity. A half-life (T1/2) of C1 activity in fluid phase at 37 degrees C was 20 min under physiologic conditions.