Ornithine Transcarbamylase (OTC) in White Blood Cells

Abstract

Summary: A radiochemical assay method of ornithine transcarbamylase (OTC) was developed using labeled carbamyl phosphate as a substrate. The enzyme activities determined by this method in peripheral white blood cells from ten normal subjects were 1.32 ± 0.95 nmoles/mg/hr and the apparent K_m's, when assayed at pH 8.5, were 6.4 mM for ornithine and 0.6 mM for carbamyl phosphate. On the contrary, the apparent K_m's of human liver OTC were 0.6 mM for ornithine and 0.12 mM for carbamyl phosphate. The average OTC activity of granulocytes was 1.0 nmoles/mg/hr, whereas that of mononuclear cells was 0.4 nmoles/mg/hr. Lymphoid cell lines were established from three normal subjects and an OTC-deficient infant. All these cell lines demonstrated no OTC activity. When arginine was removed from the medium and replaced by ornithine, the lymphoid cells were unable to grow in culture. On autoradiography, the lymphoid cells showed labeling at incubation in the presence of ¹⁴C-citrulline, but not with ¹⁴C-ornithine. Speculation: The wide range of OTC activities observed in white blood cells from normal subjects might be due to a difference of the enzyme activities between granulocytes and mononuclear cells and in turn due to an individual difference of the two cell population. OTC is either absent or inactivated in lymphoid cell lines, when grown in a culture medium with arginine. The enzyme of the peripheral white blood cells might be of a different genetic origin from that of the liver.