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Evidence for Somato statin Precursors in Human Stomach, Placenta, and Amniotic Fluid^*

DAVID FITZ-PATRICK and YOGESH C. PATEI

Fraser Laboratories, Department of Medicine, Neurology and Neurosurgery, Royal Victoria Hospital, and Montreal Neurological Institute, McGill University, Montreal Quebec H3A 1A1, Canada

Address all correspondence and requests for reprints to: Dr. Y. C. Patel, Room M3-10, Royal Victoria Hospital, 687 Pine Avenue West, Montreal, Quebec H3A 1A1, Canada.

Human stomach, placenta, and amniotic fluid have previously been shown to contain immunoreactive somatostatin (IRS). The present studies were undertaken to further characterize this IRS. Gel chromatography of amniotic fluid revealed only one peak of somatostatin-like immunoreactivity (SLI; mol wt, 15,000) regardless of gestational age. Extracts of human fetal stomach contained three peaks of SLI: 87% of the total IRS coeluted with synthetic tetradecapeptide somatostatin (SRIF), 12% coeluted with synthetic somatostatin-28 (S-28), and 4% coeluted with amniotic fluid SLI. Extracts of 9- to 13-weekold placentas contained 38.9 ± 5.3 pg IRS/mg protein (range, 21–62 pg IRS/mg protein). Chromatography revealed that 57% of the total IRS coeluted with SRIF, 19% coeluted with S-28, and 23% eluted in a position indicating a molecular weight of 12,000. Serial dilutions of amniotic fluid SLI and material from each peak of stomach and placental SLI showed parallelism with synthetic SRIF. Treatment with 8 M urea and dithiothreitol did not convert any of these SLIs to smaller immunoreactive forms. Incubation of purified amniotic fluid SLI with 1% (wt/wt) L- (tosylamido 2-phenyl)ethyl chloromethyl ketone-trypsin for 90 min resulted in partial conversion to immunoreactive material coeluting with SRIF. When synthetic S-28 was incubated in fresh amniotic fluid at 37 C, it was rapidly degraded (t_1/2 25 min). These studies indicate that human amniotic fluid IRS is composed of 15K SLI only, whereas human stomach and placental IRS are heterogeneous, comprising SRIF as well as larger forms of SLI which probably represent SRIF precursors.

^* This work was supported by grants from the Canadian Medical Research Council and the USPHS (AM-21373). Parts of this work were presented at the 72nd Annual Meeting of the American Society for Clinical Investigation, Washington D.C., May 1980.

Research Fellow, Juvenile Diabetes Foundation. Present address: Department of Endocrinology, Straub Clinic and Hospital, 888 South King Street, Honolulu, Hawaii 96813.

Scholar, Medical Research Council of Canada.

Received June 16, 1980.