The nature of pituitary large growth hormone as studied by immunoabsorption

Porcine pituitaries were homogenized in neutral pH buffer and the supernatant layer after centrifugation was fractionated by gel filtration on Sephadex G-200. The void volume peak contained 0.2-0.5% of the immunoreactive growth hormone (large GH), the remainder (small GH) eluted at Kd 0.6. The isolation of large GH was attempted with anti-GH coupled to Sepharose 2B: of the immunoreactive large GH 50-60% was absorbed to a large excess of anti-GH Sepharose during an 18 h incubation at 4 degrees C. Over 80% of the absorbed immunoreactivity could be eluted with neutral 6M-sodium thiocyanate at 4 degrees C. The eluate, when rechromatographed, contained small GH, RNA which had absorbed to anti-GH Sepharose remained in the void volume on rechromatography. RNA was also absorbed when Sepharose-coupled bovine serum gamma-globulin was substituted for anti-GH Sepharose or when rat liver was substituted for porcine pituitaries. No RNA was absorbed when large GH was converted into small GH with 6M-NaSCN and rechromatographed before immunoabsorption. However, significant absorption of prolactin was found. After a two-hour incubation of porcine pituitary slices with [3H]leucine, GH dissociated from large GH had a specific activity 10-100 times that of small GH but appeared similar to it by gel chromatography and polyacrylamide gel electrophoresis. We conclude that pituitary large GH represents a different metabolic pool from pituitary small GH and may be newly synthesized small GH not yet incorporated into granules, thus being susceptible to non-specific binding to RNA during the extraction procedure.