Participation of the lone tryptophan residue of rat α-foetoprotein in its drug-binding sites. Comparison with rat serum albumin

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Abstract
The participation in drug binding of the lone tryptophan residue of rat .alpha.-foetoprotein (.alpha.-FP) and serum albumin, the two main transport proteins of foetal serum, has been studied by two different techniques. Firstly, the effect on phenylbutazone and warfarin binding of the chemical derivatization of the lone tryptophan residue of both proteins by 2-nitrophenylsulphonyl chloride (NPS) was studied. Secondly, the effect of phenylbutazone binding on the intrinsic fluorescence of the tryptophan residue of rat .alpha.-FP and albumin was investigated. The specific modification of the proteins by NPS did not affect the binding of warfarin by rat .alpha.-FP and albumin, but greatly decreased the affinity of the high-affinity sites of rat .alpha.-FP for phenylbutazone, though the numbers of these sites were not significantly changed. However, for albumin a similar decrease in the affinity constant appeared to be due to the reaction conditions. The spectrofluorimetric studies showed that the lone tryptophan residue of .alpha.-FP and albumin was quenched by phenylbutazone binding, and the quenching paralleled the fractional saturation of the high-affinity site only in the case of albumin. The effect of phenylbutazone binding on the intrinsic fluorescence of rat .alpha.-FP indicated that the lone tryptophan residue of this foetal protein is not in the same molecular environment as that of albumin, not participating directly in the high-affinity site for phenylbutazone, and the effect may be via some induced conformational change in rat .alpha.-FP. These results also confirm our previous suggestion that the high-affinity sites for phenylbutazone and warfarin are different on the rat .alpha.-FP molecule. The results seem to indicate that this also the case for albumin, but confirmation is necessary.