Circular dichroism of gonadotropin recombinants

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Abstract
Circular dichroic spectra have been determined for ovine and porcine lutropin, human choriogonadotropin, and their subunits (α and β). All nine homologous and heterologous αβ recombinants were prepared from the purified subunits and characterized by circular dichroic spectroscopy over the wavelength interval of 200–320 nm. Above 250 nm, the spectra of all recombinants were similar and more closely resembled the spectra of the β subunits than of the α subunits. Circular dichroic difference spectra above 250 nm, derived by subtracting the sum of the subunit spectra from the appropriate recombinant spectrum, were characterized by a negative extremum near 280 nm. For a common α subunit, the magnitude of this difference extremum was in the order choriogonadotropin β > ovine lutropin β > porcine lutropin β. Interestingly, this was the same order that we found for biological activity based upon an in vitro assay [Strickland, T.W. & Puett, D. (1981) Endocrinology109, 1933–1942]. The lutropin α‐containing recombinants exhibit a positive band at 235 nm which arises from the 233 nm band characteristic of the lutropin α subunit. Between 210–230 nm, the circular dichroic spectra of the recombinants are quite similar. Even at lower wavelengths only the human choriogonadotropin α subunit‐containing recombinants exhibit differences. Aside from the overall general similarity in the circular dichroic spectra of these recombinants, the most striking observation is that the spectrum of a given recombinant is quite distinct from the additive spectra of the constituent α and β subunits. This finding, in conjunction with spectral data and chemical evidence from other laboratories, supports the concept that the subunit conformations in an αβ complex differ from those of the free subunits.