Isoelectric properties, lectin binding characteristics and biological activities of neuraminidase-treated rat LH components.

The present study was performed to evaluate the different carbohydrate structure of rat LH isoelectric components related to their intrinsic biological activities. Terminal sialic acid residues were essential to the formation of multiple LH components observed in the isoelectric focussing profile, which was proved by their interaction with Ricinus communis agglutinin-120 following neuraminidase treatment, and the conversion of component F (pI, 10.0) to less alkaline components after incubation with liver Golgi membrane fraction in the presence of CMP-NeuNAc. The affinity studies using lentil lectin indicated that component F was not an asialo form of component A (pI 8.4). The serial removal of sialic acid residues from these components led to increases in the steroidogenic activity, owing to increases in the activation of the receptor-adenylate cyclase system. The enhancement of the steroidogenic activity by desialylation was very great in component A'(pI, 8.0) (751% increase), and decreased with increasing pI. It can be concluded that the different biological potencies of intact LH components are attributable principally to terminal sialic acid residues. However, the peripheral chains of asialo oligosaccharides of less alkaline components (pI, 8.0, 8.4) seem to prevent the maximal cellular responses, since their desialylated forms did not attain the maximum activity.