Absence of rapid exchange component in a low-affinity carrier transport

A previous study showed that human red blood cells equilibrate much less rapidly with D-glucose at moderately high concentrations than with C(14)-glucose added after the net movement is completed. This had been predicted from a simple reversible mobile-carrier mediated-transport model system suggested by the net monosaccharide transport kinetics in these cells, but is also consistent with the more complex models proposed for certain active transport systems to account for elevation of tracer fluxes of even low-affinity "substrates" when their trans-concentration is raised. The simple model predicts, however, that with any sugar showing a much lower apparent affinity for the reactive sites, such as D-ribose, this phenomenon would not be observed, and tracer equilibration should proceed at approximately the same rate as net uptake. The latter expectation was confirmed experimentally by analyses of the ribose, or radioactivity, content of washed red cells sampled serially during incubation with ribose or C(14)-ribose in the appropriate mixtures. The tracer ribose movement showed no evidence of a relatively rapid exchange component. The relative rapidity of glucose tracer uptake into cells preloaded with ordinary glucose may therefore more readily be attributed simply to depression of tracer efflux by competition for the saturated reactive sites, than to any action of the trans-concentration on the influx by way of a coupled exchange process.

INFLUENCE OF DEOXYRIBONUCLEIC ACID DEGRADATION PRODUCTS AND ORTHOPHOSPHATE ON DEOXYNUCLEOTIDE KINASE ACTIVITY AND DEOXYRIBONUCLEIC ACID SYNTHESIS IN PNEUMOCOCCUS TYPE 3

Firshein, William (Wesleyan University, Middletown, Conn.). Influence of deoxyribonucleic acid degradation products and orthophosphate on deoxynucleotide kinase activity and deoxyribonucleic acid synthesis in pneumococcus type III. J. Bacteriol. 90:327-336. 1965.-An oligodeoxynucleotide fraction derived from a deoxyribonuclease-treated calf-thymus deoxyribonucleic acid (DNA) can enhance the activity of deoxycytidylic acid (dCMP) and deoxyguanylic acid (dGMP) kinases in cell suspensions of type III pneumococci. High levels of orthophosphate can produce similar effects. For part of the incubation period, the activity of dCMP and dGMP kinases is very low or undetectable in unsupplemented-cell suspensions of pneumococci. In contrast, the remaining kinases, deoxyadenylic acid and thymidylic acid, which are present in ample amounts in control and supplemented cells throughout the incubation period, are unaffected by the addition of oligodeoxynucleotides and orthophosphate. The stimulation of kinase activity is amino acid-dependent and can be abolished by adding chloramphenicol. When the oligodeoxynucleotide fraction and orthophosphate are further supplemented with all eight of the naturally occurring deoxynucleosides and deoxynucleotides (which do not affect kinase activity), a preferential enhancement of DNA synthesis occurs in comparison with cell growth or protein synthesis. Addition of deoxynucleosides and deoxynucleotides to unsupplemented cells produces only a slight increase in DNA synthesis. The preferential enhancement of DNA synthesis can be prevented by adding chloramphenicol at a certain time during incubation.

EFFECT OF AMINO ACIDS ON STEADY-STATE GROWTH OF A GROUP A HEMOLYTIC STREPTOCOCCUS

Davies, Helen C. (University of Pennsylvania, Philadelphia), Fred Karush, and Joanne H. Rudd. Effect of amino acids on steady-state growth of a group A hemolytic streptococcus. J. Bacteriol. 89:421-427. 1965.-A study has been made of amino acid utilization by a strain of type 4, group A streptococcus growing under steady-state conditions in a continuous-culture device and supplied with a completely synthetic medium. At a fixed growth rate, corresponding to a generation of time of 84 min, and with the pH maintained constant at 7.4, the bacterial turbidity was made dependent on the concentration of one of the amino acids of the defined medium. Under these conditions, the extracellular concentration of the limiting amino acid is fixed by the preset growth rate. The steady-state concentration of each of 14 essential l-amino acids was measured by means of C(14)-labeled amino acids in such limited cultures. At approximately equal turbidities, these concentrations ranged from 1.6 x 10(-6)m for methionine to 4.3 x 10(-4)m for glutamic acid. The rates of utilization of the amino acids ranged from 26 mmumoles per mg (dry weight) of bacteria per hr for histidine to 310 mmumoles per mg (dry weight) of bacteria per hr for glutamic acid. The percentage of the limiting amino acid used varied from 95% for threonine and methionine to 43% for gluamic acid. The rate of utilization of the limiting amino acid at unit concentration (tmoles per gram per hour per m) differed by a factor of 27 between extremes. These observations reflect the variation in the capacity of this streptococcal cell to take up and use different amino acids.

EFFECT OF INTESTINAL BYPASS ON CHOLESTEROL ABSORPTION AND BLOOD LEVELS IN THE RABBIT

A chronic in vivo preparation for the study of cholesterol absorption is described. Ten rabbits which had undergone ileocecal valvuloplasty, ten rabbits subsequent to upper small intestinal bypass and ileocecal valvuloplasty, and ten rabbits with lower small intestinal bypass (including the ileocecal valve), are compared to ten normal controls, on the basis of blood radioactivity, and whole blood cholesterol levels, following a force-fed standard test dose of cholesterol-4-C14, 10 mg, 0.4 µc/mg. The conclusions arrived at from the data are that the entire small intestine is capable of absorbing cholesterol, that bypass of either the upper or the lower half of the small intestine results in a marked (> 80%) reduction in the blood radioactivity subsequent to the oral test dose, and that there is a functional difference between the upper and the lower small intestinal bypass groups. Transit time in the small intestine appears to play a major role in the amount of cholesterol absorbed. To achieve both a marked lowering of cholesterol absorption and circulating cholesterol levels, the most efficacious surgical site of bypass of the small intestine is the ileum.

CO-2 FORMED FROM ENDOGENOUS AND EXOGENOUS SUBSTRATES IN PERFUSED RAT HEART

Measurements of total CO2 and C14O2 production by the isolated, perfused rat heart showed the extent of respiration dependent on endogenous and exogenous substrates. Glucose, acetate, and palmitate, but not glutamic acid or leucine, contributed appreciably to respiration. With optimal glucose and palmitate oxidation endogenous substrates accounted for over half of CO2. Endogenous substrates were adequate to maintain respiration for 30 but not for 60 min. Dependence of respiration on lipids was suggested by the similarity of metabolic CO2 in glycogen-depleted hearts perfused with no added substrate, with palmitate-1-C14, and with glucose-U-C14 plus palmitate. A reduction in C14O2 and in total CO2 formation with glucose-U-C14 in the presence of palmitate provided further support for the preferential oxidation of fatty acids. The Vo2, estimated from the total and C14O2, and an assumed respiratory quotient, indicated adequate oxygenation of the heart in the closed recirculated system. The necessity of maintaining an adequate coronary artery flow rate was indicated by an altered pattern of cell metabolism in hearts with reduced coronary perfusion.