Bovine luteinizing hormone. Circular dichroism and thermal difference spectra.

The circular dichroism of bovine luteinizing hormone in the far ultraviolet (200-240 nm) and near ultraviolet (240-320 nm) is reported as a function of pH, reduction and denaturation. The significance of side-chain ellipticity in the native and fully randomised hormone is critically examined, using denatured and reduced ribonuclease and insulin as bases for comparison. Disulphide groups make no measurable contribution to the side-chain ellipticity. Judged by the spectra of the subunits both isolated and recombined, those treatments which promote subunit disociation without causing chain unfolding, reversibly decrease the ellipticity in the near ultra-violet with minimal effect in the far ultraviolet. Thermal perturbation difference spectroscopy of bovine luteinizing hormone and its subunits shows that, in common with the ovine and porcine hormones, there are two tyrosine residues located at the interface between the subunits and inaccessible to water. Only two or three of the five water-accessible tyrosine residues are reactive to N-acetylimidazole.