Limits to Catalysis by Ribonuclease A

Bovine pancreatic ribonuclease A (RNase A) catalyzes the cleavage of the P-O(5') bond in RNA. Although this enzyme has been the object of much landmark work in bioorganic chemistry, the nature of its rate-limiting transition state and its catalytic rate enhancement had been unknown. Here, the value of k(cat)/K(m) for the cleavage of UpA by wild-type RNase A was found to be inversely related to the concentration of added glycerol. In contrast, the values of k(cat)/K(m) for the cleavage of UpA by a sluggish mutant of RNase A and the cleavage of the poor substrate UpOC(6)H(4)-p-NO(2) by wild-type RNase A were found to be independent of glycerol concentration. Yet, UpA cleavage by the wild-type and mutant enzymes was found to have the same dependence on sucrose concentration, indicating that catalysis of UpA cleavage by RNase A is limited by desolvation. The rate of UpA cleavage by RNase A is maximal at pH 6.0, where k(cat) = 1.4 × 10(3) s(-1) and k(cat)/K(m) = 2.3 × 10(6) M(-1)s(-1) at 25°C. At pH 6.0 and 25°C, the uncatalyzed rate of [5,6-(3)H]Up[3,5,8-(3)H]A cleavage was found to be k(uncat) = 5 × 10(-9) s(-1) (t(1/2) = 4 years). Thus, RNase A enhances the rate of UpA cleavage by 3 × 10(11)-fold by binding to the transition state for P-O(5') bond cleavage with a dissociation constant of <2 × 10(-15) M.     

Sensitivity to stress in the bivalve Macoma balthica from the most northern (Arctic) to the most southern (French) populations: low sensitivity in Arctic populations because of genetic adaptations?

The stress sensitivity, determined in copper exposureexperiments and in survival in air tests, and thegenetic structure, measured by means of isoenzymeelectrophoresis, were assessed in populations of theBaltic clam Macoma balthica (L.) from itssouthern to its northern distribution limit, in orderto test the hypotheses that near the distributionlimit the clams would be more stress sensitive andwould have a lower genetic variability. Thepopulations in west and north Europe show a stronggenetic resemblance. The populations in the sub-ArcticWhite Sea are genetically slightly different, and showa low stress sensitivity. The populations in theArctic Pechora Sea are genetically very distant fromthe other populations, and show the lowest stresssensitivity. Near the southern distribution limit, inagreement with the hypotheses, genetic variability islow and stress sensitivity high. On the other hand, incontrast to expectation, near the northerndistribution limit, in the populations of the PechoraSea, the genetic variability was higher, thus notreduced, and the stress sensitivity was low comparedto all other populations. Yet, it remains a questionif such is due to gradual physiologicalacclimatization (and ongoing differential selection)or to genetic adaptation.     

Mechanism of action of dialkylaminoethylthiol derivatives of pyrimidine and quinazoline

A study was made of acute toxicity and radioprotective efficiency of some heterocyclic isothiuronium analogues and their ability to change the oxygen uptake by the organism. The rate of hydrolysis of the preparations in vitro was determined. A correlation was found between the above-mentioned indices. On the basis of the data obtained it was concluded that aminothiol, formed as the result of enzymic digestion in the organism, was responsible for the radioprotective effect of the compounds under study since it reduced the oxygen uptake by the organism.     

Genetic analysis of an osmotic sensitive

Summary The genetic analysis of VY1160 sorbitol dependent, osmotic sensitive yeast mutant led to the identification of three different nuclear recessive mutations. Two of them, designated sorb^- and ts1 are closely linked to one another. The mutation sorb^- determines the lysis, while the mutation tsl increases the ability for lysis of the sorbitol dependent cells. The third mutation ts2 segregates independently from the other two and confers the sensitivity of VY1160 mutant cells towards rifampicin.     

Protective activity of a cell-free Klebsiella vaccine in relation to different Klebsiella pneumoniae serovars

The capacity of dried Klebsiella cell-free vaccine, obtained from strain No. 204 by the disintegration of microbial cells with hydroxylamine, for protecting mice from Klebsiella septic infection caused by the homologous serovar and 9 heterologous serovars of K. pneumoniae was studied. The newly developed preparation was found capable of stimulating immunity not only to the homologous K. pneumoniae serovar, but also to other K. pneumoniae heterologous serovars: K1, K9, K11, K16, K20, K61. The protective capacity of the preparation with respect to these serovars was not inferior to that of the vaccines prepared by the same method from the corresponding homologous strains. The capacity of the vaccine to protect mice from Klebsiella sepsis was manifested irrespective of the virulence of the strains used for challenge.     

Tissue-specific mammary gland antigen identified by monoclonal antibodies

A new antigen of mammary tumor cells has been revealed using monoclonal antibodies. A similar antigen was detected also in certain types of secretory epithelia of oesophagus, stomach, salivary gland, and embryonic large intestine. Upon malignization, antigen persists in invasive ductal cancer of mammary gland and is expressed in cancers of ovary and rectum. The molecular weight of the antigen is greater than 2,000 kD.     

Structural and functional features of caudatocortical connections

Efferent connections of the caudate nucleus were studied experimentally in 13 cats after electrolytic destruction of various parts of it. Preparations were stained by the methods of Nauta, Knook, and Fink-Heimer. Direct caudatocortical connections with a definite topical organization were discovered: a ventrodorsal projection in the cortex corresponds to a rostrocaudal and ventrodorsal distribution of neurons in the caudate nucleus. Striatocortical fibers are few in number and very thin. Their predeterminals were found in both the deep and the superficial layers of the cortex on granule cells and pyramidal cells of different sizes. The results are in agreement with those of most of the recent physiological investigations indicating a close functional connection between the caudate nucleus and the cortex.