Abstract:
It has previously been recognized that the major biochemical toxicity
induced by sulphide is due to an inhibition of cytochrome ~ oxidase.
Inhibition of this enzyme occurs at 30°C and pH 7.4 with a Ki of approximately
0.2 ~M, and a kon of 104 M-1 s-l, under catalytic conditions. However, the
equimo1ar mixture of sulphide and the enzyme shows identical catalytic
behaviour to that of the native enzyme. This cannot readily be attributed
to rapid dissociation of sulphide, as both spectroscopic and plot analysis
indicate the koff value is low. The addition of stoichiometric sulphide to
the resting oxidized enzyme gives rise to the appearance of a low-spin
ferric-type spectrum not identical with that seen on the addition of excess
sulphide to the enzyme aerobically. Sulphide added to the enzyme anaerobically
gives rise to another low-spin, probably largely ferric, form which upon
admission of oxygen is then converted into a 607 nm species closely resembling
Compound C. The 607 nm form is probably the precursor of oxyferricytochrome
aa3. The addition of successive a1iquots of Na2S solution to the enzyme
induces initial uptake of approximately 3 moles of oxygen per mole of the
enzyme. Thus, it is concluded that:
1. the initial product of sulphide-cytochrome c oxidase interaction is not
an inhibited form of the enzyme, but the low-spin (oxyferri) ~3+~+ species;
2. a subsequent step in which sulphide reduces cytochrome ~ occurs;
3. the final inhibitory step, in which a further molecule of sulphide binds
to the cytochrome ~ iron centre in the cytochrome ~2+~+ species, gives the
cytochrome a2+~+-H2S form which is a half-reduced fully inhibited species;4. a 607 run form of the enzyme is produced which may be converted into a
catalytically active low-spin (oxyferri) state; and therefore
5. liganded sulphide may be able to reduce the cytochrome 33 -Cu centre
without securing the prior reduction of the cytochrome a_ haem group or the
Cud centre associated with it.
