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Further Studies of Homozygous C3 Deficiency

Children's Hospital Medical Center, Boston, Mass. and University of the Witwatersrand, Johannesburg, South Africa.

Abstract

Studies of the inheritance of C3 types and partial deficiency of C3 in over 100 relatives of a previously reported Afrikaner child with virtually no serum C3 revealed that she was homozygous for a C3 gene producing little or no C3, C3^-. Metabolic studies with purified radiolabeled C3 in this patient showed a mildly elevated fractional catabolic rate and a markedly reduced synthesis rate for C3. There was also a mild increase in the rate of conversion in vitro of labeled C3 added to her serum and incubated at 37°C for 1 hr. The intradermal injection of Cs, which produces a marked increase in vasopermeability in the skin of normal subjects, produced no definite change in the patient, possibly implicating C3 or a protein in the alternative pathway as the normal mediator of this response. On incubating the patient's serum with purified cobra venom factor in the presence of unsensitized guinea pig erythrocytes, C5 inactivation and normal lysis of the red cells occurred, consistent with the venom factor being altered cobra C3.