Many methods have been described in the literature for determining the activity of ribonuclease (McDonald, 1955); most of these employ ribonucleic acid as substrate and are not suitable for kinetic investigations because it is not clear which

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Ross, Mathias & Rabin, 1962) provide evidence that the catalytic site of ribonuclease contains two imidazole groups; one of these is required in the acid form and the other in the base form. For the hydrolysis of cytidine 2',3'-phosphate, the shifts in the pK values of the active site on binding the substrate indicate that the acid site, but not the base site, is involved in binding the substrate. The acid site is probably hydrogen-bonded to the 2'oxygen atom of the cyclic phosphate and this accounts for the much stronger binding of cytidine 2'-phosphate compared with that of the 3'phosphate. There must be additional sites of enzyme-substrate binding which do not ionize over the pH range 4-8-5, since K., is not identically

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Holton for his collaboration in the study of changes in the extinction of heart-muscle preparation during activation. It is a pleasure to thank Miss Valerie Francis for her technical assistance.

Partially purified kinin, a polypeptide in wasp venom, has been found to be a potent smoothmuscle stimulating and hypotensive agent. Such a preparation was 10 to 100 times more effective than histamine in enhancing capillary permeability on intradermal injection, and 10 times more effective than acetylcholine in evoking pain on a cutaneous blister base. Silva, 1955) are potent polypeptides, with many properties in common. All are known to produce a characteristic delayed slow contraction of the isolated guinea-pig ileum, and to lower markedly the arterial blood pressure of the rabbit and cat on intravenous injection. They are stable in neutral or acidic, but not in alkaline, solution; they are insoluble in anhydrous acetone or ether, and are rapidly inactivated by chymotrypsin. Kinin occurs in large quantities, together with histamine and 5-hydroxytryptamine, in the venom of the common wasp (V. vulgaris); kallidin and bradykinin are released from the a1 globulin fraction of serum by the action of kallikrein and trypsin (or B. jararaca venom) respectively. In view of their similar properties it was of interest to extend previous studies of these three substances and to compare their properties under the same conditions. Preliminary results of such a comparison have been reported to the Physiological Society (Holdstock, Mathias and Schachter, 1956).In the present experiments kinin has been further purified and characterized chemically and pharmacologically. It has been found to be extremely effective in enhancing capillary permeability and in evoking cutaneous pain on application to a blister base. Kallidin and bradykinin were also shown to increase capillary permeability. The different methods of preparing kallidin and bradykinin were compared, and some modifications of previous methods are described.Despite their many common properties, kinin has been definitely distinguished from kallidin and bradykinin. The latter two substances, however, were not successfully distinguished by pharmacological or chemical tests, and must be closely similar, or possibly identical, compounds. MEMODSIsolated Smooth Muscle and Arterial Blood Pressure. Preparations of guinea-pig ileum, rabbit intestine, and rat uterus were suspended in Tyrode solution in an 18 ml. bath. Guinea-pig and rabbit intestine were kept at 34 to 36°C. and rat uterus (anoestrous) at 26 to 280 C., the latter temperature being required to eliminate spontaneous activity in this preparation. Atropine (0.2 ,ug.) and mepyramine (0.4 jsg.) were added to the bath before each test, except when assaying for histamine, in which case mepyramine was omitted. Usually the polypeptide was in contact with the tissue for 60 sec.Dogs were anaesthetized by intravenous injection of a mixture of chloralose (50 mg./kg.) and urethane (500 mg./kg.); rabbits received pentobarbitone sodium (30 mg./kg.) intravenously, supplemented by ether. The substances tested were injected into the femoral vein and the arterial blood pressure was recorded with a mercury manometer from a carotid a...

1. The nuclei of the cells of the whole rat brain have been fractionated in a B-XIV zonal rotor with a discontinuous gradient of sucrose. Five fractions were obtained. Zone (I) contained neuronal nuclei (70%) and astrocytic nuclei (23%). Zone (II) contained astrocytic nuclei (81%) and neuronal nuclei (15%). Zone (III) contained astrocytic nuclei (84%) and oligodendrocytic nuclei (15%). Zone (IV) contained oligodendrocytic nuclei (92%) and zone (V) contained only oligodendrocytic nuclei. 2. The content of DNA, RNA and protein per nucleus was determined for each zone. Although the amount of DNA per nucleus is constant (7pg) the RNA varies from 4.5 to 2.5pg/nucleus and the protein from 38 to 17.6pg/nucleus. The neuronal nuclei have the greatest amounts of protein. The oligodendrocytic nuclei have the least content of RNA and protein. 3. The effects of pH, ionic strength, and Mg(2+) and Mn(2+) concentration on the activity of the nuclear system for synthesis in vitro of RNA have been investigated for unfractionated nuclei. From these studies a standard set of conditions for the assay of nuclear RNA polymerase has been established. 4. The activity of the RNA polymerase in each of the zonal fractions has been determined in the presence and in the absence of alpha-amanitin. Zone (II) is the most active, followed by zone (I). The nuclei of zones (IV) and (V) have comparable activity, which is 40% of that of zone (II). 5. The extent of incorporation of each of the four labelled nucleoside triphosphates by the nuclei from each zone has been measured. These values have been used to calculate the base composition of the RNA synthesized in vitro in each class of nucleus. 6. The effect of changes in the condition of assay of RNA polymerase in the different classes of nuclei has been investigated. Significant differences in the response to concentrations of metal ions and ammonium sulphate have been observed. 7. Homopolymer formation in each zone of brain nuclei has been determined. The extent of formation of the four homopolymers roughly parallels the RNA polymerase activity.

In the degradation of ribonucleic acid by pancreatic,-ribpnuclease, uridine and cytidine 2':3'