Secretin

SECRETIN, a substance of unclear chemical composition and structure which, upon entering the blood as a result of absorption from the digestive tract or through artificial parenteral administration, causes the secretion of digestive glands. A distinction is made between gastric secretin ("gastrin") and duodenal secretin (see Digestion). Obtaining duodenal secretin. The method of Bayliss and Starling. The mucosa and submucosa of the small intestine from the pylorus to almost the ileum is scraped off and ground with a double quantity of n/10 HCl. The resulting mass is heated to 100° with careful stirring and filtered. The light-yellow filtrate, which is a mixture of very diverse substances, is used for injections after neutralization. The method of Takacs (Takacs, 1927-28). The mucosa of the duodenum and the upper 1/3 of the jejunum is scraped off in as fresh a state as possible and ground with a small amount of 0.4% HCl. After adding more HCl, the resulting mushy mass is boiled for 15 minutes, neutralizing with alkali during boiling (proteins precipitate). The filtrate is evaporated and dried. The dry residue, ground into a powder, is dissolved in the smallest possible amount of 75% alcohol. From the resulting solution, secretin is precipitated by adding a triple amount of acetone. After separating the precipitated sediment, it is dissolved in water and treated with picric acid until the formation of sediment ceases. After two days of settling, the liquid is filtered, the residue on the filter is thoroughly washed with water, dried, and processed by grinding into a powder with a tenfold amount of alcohol, a fivefold amount of absolute alcohol acidified with HCl, and finally a twofold amount of acetone. The acetone-alcohol precipitate is filtered off and freed from the admixture of picric acid by washing with acetone and ether. In the resulting preparation, it is possible to significantly reduce the protein content (from 15% nitrogen to 3%) by precipitation with table salt and subsequent treatment with zinc hydroxide. In the opinion of Takacs, it is best to purify secretin preparations by the method of ultrafiltration: a 10% solution of the preparation obtained by the picrate method is subjected to ultrafiltration through a double or triple layer of a collodion filter (pressure 1 atm). The colorless filtrate is evaporated to dryness and ground into a powder. 0.015-0.04 mg of such secretin per 1 kg of animal weight causes the same secretion as 4 mg of the preparation obtained by the picrate method. From 100 kg of duodenal mucosa, it is possible to obtain approx. 300 g of substance by the method of Bayliss and Starling, approx. 40 g when using the picrate method, and 0.15-0.3 g (depending on the quality of the filter) using ultrafiltration. The activity of the preparation increases as it is purified. Chemical properties of secretin. Most authors consider secretin to be a protein-like substance of the secondary albumose type. Secretin preparations give a biuret reaction, a reaction with sulfosalicylic acid, and sometimes a Millon reaction. With insufficient purification, secretin contains phosphoric acid. Secretin is highly soluble in water and does not dissolve in acetone or ether. It is sparingly soluble in alcohol. Proteolytic enzymes of the gastrointestinal tract destroy secretin. When heated with acids and alkalis (even diluted ones), purified preparations are rapidly destroyed; unpurified secretin is resistant to heating in both neutral and acidic or weakly alkaline environments. Secretin is not an enzyme. Secretin and enterokinase are completely different substances (Babkin).