Tetrahydrobiopterin, a Cofactor in Mitochondrial Electron Transfer

Abstract

Biopterin concentration was determined by the microbiological Crithidia test in sonified rat liver mitochondria (0.03 mM, 0.4 nmol/mg protein) and in high‐speed supernatant of rat livers (0.015 mM, 0.15 nmol/mg protein). Respiration of intact mitochondria is not influenced by presence of 7,8‐dihydroneopterin. Tetrahydropterins (biopterin, neopterin, 6,7‐dimethylpterin), on the other hand, lead to an elevated net oxygen consumption in metabolic state 3 as well as in state 4 rat liver mitochondria, but ATP formation is diminished in the presence of tetrahydropterin. In state 3, high tetrahydropterin concentrations (> 0.4 mM) seem to inhibit respiration; preincubation with reduced pterin shifts the inhibitory concentration to about 0.25 mM. Used as unique substrate, tetrahydropterins considerably activate respiration under state 4 conditions; at concentrations of 0.5–0.6 mM, saturation is gained and no further activation is possible with still higher concentrations. Low temperature‐difference spectra demonstrate a reduction of cytochromes c and a/a 3 by tetrahydropterins, and no reduction of cytochrome b. Reduction of cytochromes is time‐dependent. This lag phase is influenced by substrates, ADP, type and concentration of tetrahydropterin.