Reductive activation of carbon tetrachloride by human haemoglobin

By Ferrara, R. - Rezzadore, M. - Cazzaro, S. - Tolando, R. - Manno, M.

The reductive metabolism of carbon tetrachlorine (CCl4) 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 430 nm 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 CCl4 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 CCl4 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 CCl4 metabolites to products which maintain a tetrapyrrolic structure but are unable to react with pyridine.