PubMed Health⌕ Search

Biomedical subjects

D S Parker

Publications and source records attributed to D S Parker.

11 recordsLinked to original sources
PubMed1978

Rabbit liver acetyl-CoA synthetase.

Acetyl-CoA synthetase (EC 6.2.1.1) was assayed in subcellular fractions of rabbit liver homogenates. The activity was located almost exclusively in the cytosol. There was no decrease in activity when butyrate or propionate (each at 5--20 mM) were added to the assay medium.

Acetate-CoA Ligase↗
PubMed1978

Uptake of substrates for milk-fat synthesis by lactating-rabbit mammary gland.

1. A cannulation technique is described for measuring arteriovenous differences across the lactating-rabbit mammary gland. 2. Analysis of milk obtained before and after surgery shows no effect of cannulation on milk constituents. 3. Results of blood analysis show significant net changes in the concentrations of glucose, acetate, 3-hydroxybutrate, triacylglycerols and non-esterified fatty acids across the mammary gland. 4. The molar proportions of individual fatty acids in both the triacylglycerol and non-esterified fatty acid fractions did not alter between the arterial and venous samples. 5. The extraction rates are compared with those obtained from other species.

Acetates↗
PubMed1977

Nuclease B. A possible precursor of nuclease A, an extracellular nuclease of Staphylococcus aureus.

During purification of nuclease (redisignated as nuclease A in the present studies) from the culture media of Staphylococcus aureus strain Foggi, three enzymacally active second species (nucleases B1, B2, and B3) were isolated as a mixture by ion exchange chromatography. Examination of the amino acid sequence of these second species indicates that nucleases B1, B2, and B3 apparently contain the same sequence as that of nuclease A with an extra sequence Ser-Gln-Thr-Asp-Asx-Gly-Val-Asx-Arg-Ser-Gly-Ser-Glu-Asp-Pro-Thr-Val-Tyr-Ser linked through a peptide bond to the NH2 terminus of the nuclease A portion. In nuclease B1 the 2 residues indicated by Asx are aspartic acid, in nuclease B2 the first and the second Asx from the NH2 terminus are aspartic acid and asparagine, respectively, and in nuclease B3 both Asx are asparagine. These second species do not contain a significant amount of carbohydrate. The extra amino acid sequence appears to be flexible and does not interfere with the ordered structure and function of the nuclease A portion. The nuclease A portion was recovered, in part, from a mixture of these nuclease B species after digestion with staphylococcal protease in the presence of ligands, deoxythymidine 3',5'-diphosphate, and calcium ion. Thus, these nuclease B species may be closely related to, if not identical with, a precursor of nuclease A. Similar second species of nuclease have been found in strain V8.

Amino Acid Sequence↗
PubMed1977

Study of equilibration of the system involving two alternative, enzymically active complementing structures simultaneously formed from two overlapping fragments of staphylococcal nuclease.

Quantitative complementation of two overlapping fragments of staphylococcal nuclease, Nuclease-(1-126) (residues 1 to 126) and Nuclease-T-(50-149) (residues 50 to 149), simultaneously forms in 1 min, two alternative, enzymically active ordered structures (types I and II) resembling nuclease (149 residues) (Taniuchi, H., and Anfinsen, C.B. (1971) J. Biol. Chem. 246, 2291-2301). We determined the ratio of type I to type II complex formed from the two fragments as a function of time, temperature, and the presence or absence of the ligands thymidine 3',5'-diphosphate and calcium ion. The ratio of type I to type II complex was determined on the basis of the quantities of their derived complexes obtained after each experiment by removing the redundant amino acid sequences by limited digestion with trypsin in the presence of ligands. The quantity of the derived complexes was estimated by quantitative determination of the component fragments separated by gel filtration. The ratio of type I to type II complex formed in 2 min after mixing the two fragments was approximately 0.3 and appears to be independent of temperature and the presence or absence of ligands. The equilibrium of the system of type I and II complexes is attained through unfolding and folding. The ratios of type I to type II complex at the apparent equilibrium state of the system at 6 and 23 degrees were approximately 1.1 and 2.4, respectively. The observations indicate that the rate of unfolding of type II complex is greater than that of type I complex at 6 degrees and increases more than that of type I complex with increasing temperature. Thus, the change of the complementing structure from type I complex with increasing temperature. Thus, the change of the complementing structure from type I to type II causes a decrease in the activation free energy, an increase in the activation enthalpy, and thereby an increase in the activation entropy of unfolding. Since the unfolded states with which type I and II complexes are in equilibrium are the same, the distribution of the population of type I and II complexes at the equilibrium state will be determined on the basis of the respective decreases in Gibbs standard free energy from the unfolded state to type I and II complexes. On this basis type I complex has a lower energy by deltaG0 = -0.05 and -0.51 kcal mol-1 at 6 and 23 degrees, respectively, than type II complex. Nevertheless, at the initial complementation the population of type I complex formed is approximately one-third that of type II complex at both 6 and 23 degrees. That is, the probability (rate) of folding is not related to the decrease in energy from the unfolded to the folded state. Using van't Hoff's equation deltaH = 7.5 kcal mol-1 and then deltaS degrees = 27 cal deg-1 mol-1 from type II to type I complex.

Amino Acid Sequence↗
PubMed1976

The determination of volatile fatty acids in the caecum of the conscious rabbit.

1. A method of obtaining dialysed samples from the caecum of the conscious rabbit is described. 2. Values for total volatile fatty acid content and for molar proportions of individual volatile fatty acids in dialysate samples were in good agreement with those obtained from caecal material. 3. The volatile fatty acid level in the caecum throughout the day was determined using two groups of animals, one group fed ad lib. and the other group on a restricted food intake. These results indicated a marked diurnal fluctuation in volatile fatty acid level in the caecum of rabbits fed once/d which was not evident in those fed ad lib.

Acetates↗
PubMed1976

The measurement of production rates of volatile fatty acids in the caecum of the conscious rabbit.

1. The decrease in specific radioactivity of individual volatile fatty acids (VFA) after a single injection of tracer was monitored. The results obtained indicated the occurrence of a first-order process. 2. Regression analysis indicated the high flux of VFA through the caecal pool, equivalent to 30% of the maintenance energy requirement of the animal. 3. Interconversion of VFA was monitored, and results indicated substantial synthesis of butyric acid from acetic acid. 4. Results were obtained from animals on two dietary regimens, and these were compared with results reported for the other species.

Acetates↗