A method for the study of the inhibition of DNA synthesis by rabbit tissue extracts

This paper describes a method to measure DNA synthesis in the jejunum. Everted rings of rat jejunum were incubated in the presence of (3H) thymidine. The specific activity of jejunal DNA was assayed at the end of incubation. This method was found to be valid when (1) incubation time was 30 min, (2) (3H) thymidine concentration was 90 ng/ml, and (3) paired flasks containing four, respectively, contiguous rings were compared. This method was used to investigate the existence of an intestinal chalone. An aqueous extract (S-105)of rabbit small intestine mucosa was prepared and tested for DNA synthesis inhibition in jejunum. A significant inhibition was noted, which was an hyperbolic function of the quantity of extract present in the media. The study of S-105 inhibition on other organs (kidney, liver, colon) failed to demonstrate organ specificity. However, aqueous extracts from intestinal organs were found to exert a greater inhibition than aqueous extracts form other organs. It is suggested that S-105 contains other nonspecific inhibitory fractions; our data prove the necessity of an extensive purification to demonstrate the existence of a specific chalone that regulates intestinal cell proliferation.