Isolation and characterization of human LH isoforms

Abstract

ABSTRACT Thirty-nine human LH (hLH) isoforms were chromatographically separated from human pituitary extracts using a mild purification procedure which consisted of preparative isoelectric focusing, high-performance ion-exchange chromatography and immobilized metal-affinity chromatography. Twenty of these hLH isoforms were characterized by LH radioreceptor assay, SDS-PAGE and amino acid analysis, and 17 were shown to be highly purified (>90% pure). The specific activities of these hLH isoforms ranged from 1980 to 38 650 IU/mg protein in terms of the 2nd IS for human pituitary LH, based on protein content as determined by amino acid analysis. hFSH and hTSH content were <0·5% and <7·8% respectively. The purity was assessed by silver staining on SDS-PAGE. Under non-reducing conditions, a single band of apparent molecular mass 23·5–24·5 kDa was observed, whereas under reducing conditions the isoforms migrated as two distinct bands, 21·1–22·4 kDa and 18·0–20·5 kDa, probably corresponding to the α and β subunits of hLH respectively. The remaining three less pure isoform preparations (70–90% pure) contained additional bands of 16 kDa and 26·3 kDa under non-reducing conditions. All isoforms showed a low molecular mass band(s) of 11–14 kDa which was <7% of stained material as assessed by densitometry. Amino acid composition of the 17 hLH isoforms was similar to the published cDNA composition of hLH. Further fractionation of one hLH isoform (hLH IIc) on reversed-phase high-performance liquid chromatography yielded four peaks identified by N-terminal sequencing as two α and two β hLH subunits identical to their cDNA-derived N-terminal sequences. No additional sequences indicative of internal clipping of hLH were observed. The two pairs of α and β subunits probably represent two separate hLH isoforms in this preparation. It was concluded that a mild purification procedure with high recoveries for the isolation of intact hLH isoforms has been developed, and 17 isoforms of high purity suitable for further biological and physicochemical characterization have been isolated. These isoform preparations are free of other contaminating proteins, but may still contain multiple hLH-related species. Journal of Endocrinology (1993) 138, 529–543