SUMMARY.Quantitative determinations of the terminal amino groups of a-casein before and after treatment with crystalline rennin were carried out using Sanger's method of labeling the free amino groups with dinitrophenyl residues. I n native a-casein the terminal amino acids were found to belysine and arginine. I n rennin treated a-casein the same endgroups can still be detected, though lysine in smaller quantities. I n addition a new terminal amino acid namely phenylalanine appears in the a-paracasein.I n addition to chromatography a new method of determining N-terminal arginine has been successfully used diiring this work. Thereby DNP-arginine is determined directly in the mixture of DNPamino acids by means of an optical method, using the colour reaction of Sakaguchi.The results suggest, that the primary reaction in the rennet curdling of milk is a specific limited proteolysis. They further lead to the conclusion that a-casein cannot be a homogeneous protein and that only a fraction of it is altered in this enzymatic reaction.Theodor-Kocher-Institut und Institut fiir organische Chemie der UniversitBt, Bern. Ausgehend von der Hypothese, dass zahlreiche biologische Wirkungen chemischer Verbindungen auf eine Beeinflussung von Fermentsystemen zuriickzufuhren sindl), seien im folgenden einige mogliche Wirkungsmechanismen biologisch aktiver Stoffe diskutiert. I m Anschluss an diese allgemeinen Betrachtungen sol1 uber das Komplexbildungsvermogen von strukturverwandten Verbindungen, von denen eine biologisch aktiv, die andere in den gleichen Testen inaktiv ist, berichtet werden.
Metallionen-Gleichgewichte und biologische