Amino-acid Sequence of Human Placental Lactogen

AN earlier report from this laboratory¹ described striking homologies in the amino-acid composition of the peptides obtained from trypsin cleavage of human placental lactogen (HPL) and human growth hormone (HGH) and established a firm biochemical basis for the similarities in biological and immunological activity of the two hormones. The intrachain disulphide bonds were placed in similar locations and the carboxyl-terminal amino-acid sequences were shown to be identical except for one out of fourteen residues². In other studies^3,4, the amino-terminal sequence was determined and noted to be homologous for eleven of the first seventeen residues. We now report the complete amino-acid sequence of HPL and a comparison with the revised structure for HGH recently reported⁴. These two protein hormones are strikingly similar in structure, being identical in 85% of the corresponding positions. Of twenty-eight amino-acid substitutions, twenty-five are favoured changes and all but two of the twenty-five can be explained in terms of a single base change in the triplet codon⁵.