摘要:

The relation between threonine deaminase activity and threonine concentration in sonic extracts of wild-type and streptomycin-dependentEscherichia coliK-12 was found to follow a hyperbolic curve. A similar relationship was obtained between enzyme activity and pyridoxal concentration. However, when serine was used as substrate, the activity–concentration curve was sigmoid, suggesting that serine may be a weaker effector of allosteric transition than threonine. The kinetic properties of the (derepressed) threonine deaminase of streptomycin-dependentE.coliK-12 were found to be similar to those of the enzyme of the wild-type K-12.It is postulated that derepression of threonine deaminase in streptomycin-dependentE.coliK-12 provides a selective advantage which permits exponential growth of this mutant in the presence ofL-valine, which is an excretory product of streptomycin-dependent microorganisms.

ISSN:0008-4018    

DOI:10.1139/o67-001

出版商:NRC Research Press    

年代:1967

数据来源: NRC

摘要:

In the presence of end product (L-isoleucine), the biosynthetic threonine deaminase in extracts of streptomycin mutants ofEscherichia coliK-12 gave a sigmoid response to increasing substrate (L-threonine) concentration. The response to inhibitor at constant substrate concentration was similar. However, the mutants showed quantitative differences in the "threshold" concentration of effector (substrate or inhibitor) required to produce the response. The smallest threshold was observed with the wild-type (streptomycin-sensitive) strain and the largest threshold with the streptomycin-resistant (indifferent) strain. The streptomycin-dependent strain was intermediate. The differences in response to effector may be caused by structural differences at the sites of bonding between subunits of the enzyme protein.These observations on feedback regulation, together with previous observations on repression in streptomycin mutants, indicate that the effects of the antibiotic are directed towards enzymes which perform a regulatory function.

ISSN:0008-4018    

DOI:10.1139/o67-002

出版商:NRC Research Press    

年代:1967

数据来源: NRC

摘要:

The thiols of the stromata of rat erythrocytes were studied in relation to the K+loss and hemolysis caused by thiol-masking agents. When stromata were isolated from erythrocytes that had been incubated with radioactive thiol-masking agents, the total number of reactive thiols in the stromata was estimated to be 0.03 μmole/109cells, or 2% of the reactive thiols in the whole cell. A fast-reacting and a slow-reacting group of stromal thiols, constituting approximately 14 and 86% respectively of the total, could be distinguished by titration withp-chloromercuribenzoate (pCMB). The K+loss and hemolysis caused bypCMB, and presumably by other thiol-masking agents, appeared to be related to inhibition of the fast-reacting group of stromal thiols. Inhibition of these thiols also facilitated removal of residual hemoglobin and other loosely associated proteins from the erythrocyte membranes in the stromal preparations, without causing a disintegration of the stromata. The above effects may all reflect a disorganization of the stromal structure that resulted from inhibition of the fast-reacting group of stromal thiols.

ISSN:0008-4018    

DOI:10.1139/o67-003

出版商:NRC Research Press    

年代:1967

数据来源: NRC

摘要:

Male weanling rats were offered diets ad libitum 18 hours daily for 4–7 days. Protein sources included whole wheat, oatmeal, corn, rye, rice, millet, pea, soya, chick-pea, lentil, lima bean, navy bean, cottonseed, peanut, sesame, egg, fish, milk, and mixtures of animal and plant proteins. Levels of plasma lysine, methionine, threonine, tryptophan, leucine, and isoleucine were determined, and plasma amino acid (PAA) scores were calculated. The limiting amino acid(s) in each diet according to PAA score was in good agreement with that reported in the literature. These data provide convincing evidence that plasma amino acid levels can be used to determine the limiting amino acid in practical diets.

ISSN:0008-4018    

DOI:10.1139/o67-004

出版商:NRC Research Press    

年代:1967

数据来源: NRC

摘要:

A cell-free extract of mucosal scrapings of rat intestine in Krebs–Ringer phosphate buffer was shown to possess high levels of deoxyribonuclease (DNase) activity. Twenty percent saturation of the extract with ammonium sulfate gave a precipitate with enzyme activity towards DNA which resembled DNase I. Mg++ions were required for its activity which was inhibited by EDTA, citrate, arsenate, and inhibitor from pigeon-crop glands. Both the crude extract and the fraction precipitated by 20% saturation with ammonium sulfate showed optimum activity near pH 6. The crude extract, however, contained some activity which did not require Mg++. Digestion of DNA with either the crude extract or the partially purified enzyme gave rise to products ranging in size from mononucleotides to oligonucleotides of 7 units (or more) which could be separated by chromatography on DEAE-cellulose. Further treatment of the hydrolysates with snake venom or spleen phosphodiesterase and phosphomonoesterase showed that the DNase precipitated by 20% saturation with ammonium sulfate gave oligonucleotides terminating in 5′-phosphate whereas the original crude extract yielded oligonucleotides with both 5′- and 3′-phosphomonoester linkages. It was concluded that the crude extract contained a second endonuclease of the DNase II type.

ISSN:0008-4018    

DOI:10.1139/o67-005

出版商:NRC Research Press    

年代:1967

数据来源: NRC

摘要:

Studies have been carried out on the incorporation of32P-labeled α-glycerophosphate (α-G32P) into the lipids of preparations obtained from rat brain cerebral hemispheres. The optimal incorporating system contains Tris and phosphate buffers (pH 7.4), MgCl2, CoA, EDTA, NaF and ATP. The incorporation of α-G32P into monophosphoinositide was stimulated by CDP-choline, CDP-ethanolamine, CDP-glycerol and CTP. These nucleotides depressed the incorporation of α-G32P into phosphatidic acid. Microsomal preparations supported a high incorporation of α-G32P into phosphatidic acid, but the incorporation into monophosphoinositide was low compared with the whole homogenate. Similar observations were made with mitochondrial preparations undergoing oxidative phosphorylation with CoA added to the medium.

ISSN:0008-4018    

DOI:10.1139/o67-006

出版商:NRC Research Press    

年代:1967

数据来源: NRC

摘要:

Previous investigations conducted in this laboratory showed a number of differences in the cytosine nucleotide requirement for the incorporation of α-glycerophosphate (α-G32P) into the monophosphoinositide of rat brain preparations compared to the pathway described by Paulus and Kennedy, where α-glycerophosphate → phosphatide acid → CDP-diglyceride → monophosphoinositide, and CTP is specifically required. Experiments were carried out with rat brain preparations to determine the nature of the mechanism whereby CDP-choline is as effective as or more effective than CTP in stimulating the incorporation of α-G32P into monophosphoinositide. Isotope dilution experiments in which unlabeled phosphatidic acid and CDP-diglyceride were used, yielded results consistent with the view that both of these compounds are intermediates in the incorporation of a-G32P into monophosphoinositide stimulated by either CTP or CDP-choline. Time-course experiments where cytosine nucleotides were added either at the beginning or after 20 minutes produced a pattern of labeling which could be fitted into the above interpretation, provided that newly formed radioactive molecules of phosphatide acid could be used selectively and CTP in some way inhibits phosphatide acid formation or accumulation. The latter could account for the observation that monophosphoinositide becomes far more actively labeled than phosphatidic acid in the presence of added CTP.

ISSN:0008-4018    

DOI:10.1139/o67-007

出版商:NRC Research Press    

年代:1967

数据来源: NRC

摘要:

Rats expired14CO2slightly faster whenDL-phenylalanine-1-14C rather thanDL-phenylalanine-3-14C was injected. An injection ofDL-fluorophenylalanine-3-14C produced more radioactive CO2than did phenylalanine-3-14C;DL-beta-thienylalanine-3-14C produced only trace amounts of14CO2but significant levels of radioactivity in the urine. When an excess of one of the nonradioactive phenylalanine analogues was injected at the same time as radioactive phenylalanine, the catabolism of the radioactive amino acid was accelerated. This was shown by an increase in the radioactivity in both the CO2and urine when beta-thienylalanine or 2-amino-3-phenylbutanoic acid was used, and in the urine alone when fluorophenylalanine was used. The incorporation of radioactive phenylalanine into tissue proteins was decreased when one of the nonradioactive phenylalanine analogues was injected simultaneously. However3the incorporation into proteins of tyrosine-14C derived from injected phenylalanine-14C was increased under these conditions.

ISSN:0008-4018    

DOI:10.1139/o67-008

出版商:NRC Research Press    

年代:1967

数据来源: NRC

摘要:

There appears to be very little information in the literature pertaining to concentrations of various air pollutants or tobacco-smoke constituents that may be toxic or nontoxic to tissue cells cultivated in vitro. We have undertaken the task of ascertaining these levels and this report records observations concerning the first few of many known pollutants.The effects of the sodium salts of 10 monocarboxylic acids (formate, acetate, propionate, butyrate, valerate, caproate, oenanthylate, caprylate, caprate, and benzoate) on several established cell lines (HeLa, strain L, human lung, human skin) were studied.All of these compounds at a concentration of 10 mg% were toxic to the cell lines tested with the exceptions of formate, acetate, and benzoate on strain L, and of valerate and caprylate on human lung.All the compounds either stimulated proliferation or had no significant effect at 1 mg% except caproate and benzoate, which were toxic to human lung and human skin cells, respectively.

ISSN:0008-4018    

DOI:10.1139/o67-009

出版商:NRC Research Press    

年代:1967

数据来源: NRC

摘要:

Adenosine-2′ diphosphate (A2'DP) and adenosine-3′ diphosphate (A3′DP) have been prepared by reaction between the corresponding mononucleotides with phosphoramidic acid in formamide. The products were isolated by ion-exchange chromatography on DEAE-cellulose and characterized by enzymic and chemical methods. A major side reaction in the synthesis is the phosphorylation of unprotected hydroxyl groups oil the nucleotides. A2′DP and A3′DP were found to be more labile than adenosine-5′ diphosphate, and to decompose at neutral pH by direct hydrolysis of the pyrophosphate bond as well as by formation of adenosine-2′,3′ cyclic phosphate. They are stable in 10% trichloroacetic acid at 0° for 10 minutes. Phosphomonoesterase degrades both to nucleosides by way of the corresponding nucleoside monophosphates. Ribonuclease T2does not attack A2′DP but degrades A3′DP to adenosine-3′ phosphate. Neither nucleotide is degraded by snake venom or spleen phosphodiesterases under conditions where oligonucleotides are completely hydrolyzed.

ISSN:0008-4018    

DOI:10.1139/o67-010

出版商:NRC Research Press    

年代:1967

数据来源: NRC