Production of all follicle-stimulating hormone isohormones from a purified preparation by neuraminidase digestion.

Graded removal of sialic acid residues from a purified preparation of rat follicle-stimulating hormone (FSH; NIADDK-FSH-I-5) by neuraminidase digestion resulted in the production of FSH isohormones with isoelectric points identical to those found within pituitary tissue. In addition, each neuraminidase-produced FSH form exhibited a radioreceptor assay:radioimmunoassay ratio similar to that of its endogenously produced counterpart. Thus, the molecular basis for FSH microheterogeneity appears to be due to the varying degree of sialylation into a common FSH protein core. We have demonstrated that the pituitary gland produces different amounts of these FSH isohormones depending upon the surrounding endocrine environment. The results of the present studies suggest the existence of biochemical mechanisms within the pituitary that influence sialic acid incorporation into FSH. These mechanisms appear to be sensitive to the surrounding hormonal milieu and serve to alter the intensity of the FSH stimulus delivered to the gonad.