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The recent identification of human placental lactogen (HPL), a polypeptide hormone which shares both biological and immunological properties with pituitary growth hormone (HGH), has led to much speculation concerning its physiological role in pregnancy and potential usefulness as a therapeutic substitute for HGH.' Although the two proteins show partial immunological identity, purified HGH causes prominent lactogenic and somatotrophic effects,2 while HPL is predominantly lactogenic." 3 4 Since the complete amino acid sequence of HGH has recently been determined,5 and studies6-8 have suggested that HPL resemblesHGH in molecular weight and amino acid composition, a detailed comparison of their chemical similarities was initiated. In the current study, each of the tryptic peptides of highly purified and biologically active HPL was isolated and its amino acid composition determined. Striking homologies in structure with most of the tryptic peptides of HGH were noted and indicated a strong biochemical basis for the physiological properties shared by the two hormones. Materials and Methods.-Purification and characterization: Partially purified HPL was produced by methods previously described6i 8 and was obtained through the courtesy of Drs. P. H. Bell and I. Ringler of Lederle Laboratories. This material was subjected to gel filtrations6 on a column of Sephadex (Pharmacia) G-100 (2.5 X 190 cm), using 0.15 M ammonium bicarbonate buffer (pH 8.0). In a typical experiment, 250 mg of protein was loaded on the column which was run at 60 cc per hour. Analytical polyacrylamide disc gel electrophoresis was performed at pH 9.3 and 4.3,9 using a Buchler Corporation analytical disc gel apparatus. Preparative polyacrylamide electrophoresis was performed with a Polyprep apparatus (Buchler Corp.), using the Tris-HCl discontinuous buffer system developed by Jovin et al.'0 The column was prerun at 35 ma (400 volts) for 1 hr to remove the persulfate, and 20 mg of sample were loaded on the column in 5 cc of buffer. The elution was performed for 10 hr at 60 cc per hour with a current of 35 ma. Biological assays: (a) Weight gain in hypophysectomized rats: Hypophysectomized male rats (Charles River) weighing an average of 100 gm were given daily subcutaneous injections with 5.3and 9.9-mg quantities of purified HPL for periods of 7, 10, and 14 days (assays were kindly performed by Dr. Charles Breuer of Lederle Laboratories).6 (b) Midpregnant rabbits were injected with HPL for 4 days in doses varying between 1 and 10 mg (peak B highly purified protein). Reactions were graded by the method proposed by Friesen.3 Immunological assays: (a) Agar gel diffusion studies by the Ouchterlony technique were performed in plastic Petri dishes using 0.7% agar Noble. (b) Radioimmunoassay studies were performed with purified HPL or growth hormone (preparation HS 612A-gift of Dr. A. E. Wilhelmi and National Pituitary Agency) which was labeled with 1131 by a modification of the method of Greenwood et al." The labeled hormones were separated from "damaged hormone" and free iodide on small columns of DEAE-cellulose. Antibodies to HPL were obtained by injection of partially purified HPL in adjuvant into guinea pigs, and growth hormone antiserum was a gift of Dr. Solomon A. Berson and the National Pituitary Agency. Bound and free hormone were separated by chromatoelectrophoresis on Whatman 3MC paper as described by Glick et al.12 or by a modification"3 of the dextran-coated charcoal method of Herbert et al.'4 Chemical studies: Purified HPL was subjected to acid hydrolysis in 6 N constant boiling HCl

Similarities in the chemical structure of human placental lactogen and pituitary growth hormone.