Reductive Activation of Carbon Tetrachloride by Human Haemoglobin

Abstract

The reductive metabolism of carbon tetrachloride (CC1₄) by human haemoglobin (Hb) was observed in vitro by absolute absorption spectra recorded under anaerobic conditions. The following results were obtained: 1) a decrease of the 430nm peak typical of free reduced Hb (Hb²⁺); 2) the formation of a shoulder of absorbance, attributable to the production of a complex between Hb²⁺ and a metabolite of CC1₄ carbon monoxide (Hb-CO); and 3) the oxidation of some Hb²⁺ to methaemoglobin (Hb³⁺). The concentration of these three forms — Hb²⁺, Hb-CO and Hb³⁺ — during anaerobic incubation of Hb with CC1₄ was calculated algebraically from the absolute spectra. CO production was then calculated from the concentration of Hb-CO, using a suitable calibration curve. Interestingly, under identical experimental conditions, a substrate-dependent loss of Hb-derived haem, but not of Hb itself nor of haem-derived porphyrin fluorescence, was measured. Preliminary HPLC studies to clarify the discrepancy and, in particular, the role and fate of the haem group, showed two substrate-dependent modified haem products. The results indicate that human Hb is able to catalyse the reductive activation of CCl₄, and suggest that, during the process, its prosthetic group haem may be modified by CC1₄ metabolites to products which maintain a tetrapyrrolic structure but are unable to react with pyridine.