Somatostatin-Like Immunoactivity and Biological Activity Is Present in Tetrahymena pyriformis, a Ciliated Protozoan*

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Abstract
Somatostatin (SRIF) in vertebrate phyla has a widespread tissue distribution, with highest concentrations in the central nervous system, gastrointestinal tract, and pancreas. To determine the phylogenetic origins of SRIF, we examined a unicellular microorganism that in the evolutionary scale antedates specialized neural elements and endocrine cells. Three batches of Tetrahymena pyriformis grown in simple medium lacking serum or macromolecules were extracted with acidethanol. SRIF-like immunoreactivity (SRIF-LI) eluted in the void volume on Sephadex G-10 gel chromatography and was retained on CM-Sephadex G-50 ion exchange chromatography at pH 10.6, exhibiting characteristics similar to those of synthetic SRIF (batch A). Peptides from the extract (batch B) were partially purified by passage through disposable octadecasilylsilica cartridges and elution with a 100 μtl/ml stepwise gradient of acetonitrile in 1 μ ul/ml trifluoroacetic acid. SRIF-LI, like SRIF, eluted in the 300 and 400μl/ml acetonitrile fractions. Reverse phase high pressure liquid chromatography (HPLC) of this material on a cyanopropyl column in 200 μul/ml acetonitrile-800 μml/ml triethylammonium formate revealed a single SRIF-LI peak which coeluted with SRIF (retention time = 6.3 min). Serial dilutions of HPLC-purified SRIF-LI showed displacement of [125 l-Ty1]SRIF binding from antibody which was parallel to that seen with synthetic SRIF. SRIF-LI in the extracts (batch C) bound to and was eluted from SRIF antibodies linked to CNBr-activated Sepharose, suggesting immunological similarity between tetrahymena SRIF-LI and synthetic SRIF. The biological activity of HPLC-purified SRIF-LI was assessed by inhibition of dibutyryl cAMP-stimulated GH secretion from cultured rat adenohypophyseal cells. Dose-related suppression of GH release by SRIF-LI paralled that by SRIF with a potency ratio of 0.96 (95% confidence limits, 1.47-0.63) and was neutralized by incubation with excess sheep anti-SRIF immunoglobulin but not by nonimmune sheep immunoglobulin. The presence of SRIFLI in this unicellular eukaryote extends the phylogenetic range of SRIF. Together with the previously reported finding of insulin, ACTH, and hCG in tetrahymena and other microorganisms, the presence of SRIF suggests that SRIF phylogenetically antedates the origins of both the neuron and the endocrine cell.