Ribonuclease activities of the developing chick pancreas

1. 1. Two ribonuclease activities are present in embryonic chick pancreas. Both activities differ in their specificity from bovine pancreatic ribonuclease. One of the activities ('Mg²⁺-ribonuclease') is due to a soluble magnesium-requiring endonuclease which has been described previously. The other activity ('neutral ribonuclease') which is bound to particles does not require metal ions and has an optimum pH of 7.2. Neutral ribonuclease activity can be partially solubilized by raising the salt concentration. It hydrolyzes RNA core, polyuridylic (poly U) and polycytidylic (poly C) acids but not polyadenylic (poly A) or polyinosinic (poly I) acids. 2. 2. The fact that Mg²⁺-ribonuclease is actively secreted by the pancreas in vitro on stimulation with carbamycholine shows that it is a digestive enzyme. The properties of neutral ribonuclease, on the other hand, suggest that it is not a digestive enzyme. 3. 3. Differential centrifugation studies on homogenates of pancreas in isotonic sucrose show that Mg²⁺-ribonuclease, like other digestive enzymes, is located mainly in the zymogen granule and soluble fractions. A large portion of the neutral ribonuclease is associated with heavy particles sedimenting at 250 × g, but is not specifically attached to nuclei. 4. 4. Developmental patterns of the activities of the two ribonucleases differ markedly. The specific activity of neutral ribonuclease reaches a maximum between 16 and 18 days of development after which it declines. The specific activity of Mg²⁺-ribonuclease increases steeply between 19 and 21 days of development and closely parallels the increase in specific activity of amylase during this period. Before 17 days of development, however, the specific activities of Mg²⁺-ribonuclease and amylase do not increase in a parallel fashion. It is suggested that digestive enzyme activities in chick pancreas increase in a non-parallel manner before 17 days of development and afterwards in a parallel manner.