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Biomedical subjects

B L Horecker

Publications and source records attributed to B L Horecker.

At least 127 records · Page 7Linked to original sources
PubMed1978

Transport and phosphorylation of hexoses in normal and Rous sarcoma virus-transformed chick embryo fibroblasts.

Effects of transformation by Rous sarcoma virus on sugar uptake and activity and the subcellular distribution of hexokinase isozymes in chick embryo fibroblasts were examined. Transformation caused a several-fold increase in the maximum velocity for uptake of 2-deoxyglucose without a significant change in Km. Cytochalasin B (CB), was used to differentiate between the effects of transformation on facilitated diffusion and the nonsaturable (CB-insensitive) mode. Transformation was found to stimulate 2-deoxyglucose transport by both mechanisms, but the increase in transport by the CB-insensitive mode was greater. Transformation enhances the activity of hexokinase, the enhancement being confined to the particulate fraction of the enzyme. Heat-inactivation and electrophoretic mobility studies showed that although hexokinase Type I is the major form in both normal and transformed fibroblasts, there is a significant increase in the proportion of the Type II isozyme in the transformed cells.

Animals↗
PubMed1978

Translation of mRNA from calf thymus in the wheat germ system: evidence for a precursor of thymosin alpha1.

When translated in the wheat germ system, mRNA from fresh calf thymus stimulates incorporation of radioactive amino acids into an acid-insoluble product, and 10--20% of the total radioactivity incorporated is precipitated with antisera to active thymosin fractions. In sodium dodecyl sulfate disc gel electrophoresis, radioactivity was recovered mainly in two peptides, corresponding to 16,000 and 11,000 daltons; the latter probably represents incomplete chains. Tryptic digests of each of these peptides yielded fragments corresponding to the sequence of residues 15--19 of thymosin alpha1; these peptides were characterized by cochromatography with digests of synthetic thymosin alpha1 and by Edman degradation. Thus, the 16,000-dalton peptide synthesized in the cell-free system appears to be q precursor of thymosin alpha1 and possibly of other peptides in the fractions isolated from calf thymus. The results support the conclusion that this peptide is synthesized in the thymus gland.

Cell-Free System↗
PubMed1977

Binding of Zn2+ to rat liver fructose-1,6-bisphosphatase and its effect on the catalytic properties.

Rat liver fructose-1,6-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) contains 12 binding sites for Zn2+ per molecule, or 3 per subunit, as determined by gel filtration and by precipitation of an insoluble Zn2+-enzyme complex. The first set of sites binds Zn2+ with very high affinity, and the binding constant for these sites could not be determined. The average values of the dissociation constants for the second and third sets of sites were approximately 0.4 and 1.5 muM, respectively. The third set of sites, having lowest affinity, appears to be identical to the binding sites for the activating cation, Mg2+, and the binding of Zn2+ to this set of sites is prevented by the addition of Mg2+. Binding of the first 4 equivalents of Zn2+ yields an enzyme of intermediate activity, while the binding of 8 equivalent results in almost complete inhibition of catalytic activity. Thus Zn2+ appears to function as both an activator and a negative allosteric regulator of fructose-1,6-bisphosphatase activity.

Allosteric Site↗
PubMed1976

Evidence for the selective release of lysosomal proteinases in fasted rabbits.

The enzyme responsible for the conversion of "neutral" to "alkaline" fructose 1,6-bisphosphatase (EC 3.1.3.11) by removal of a 7000 dalton peptide (converting enzyme, Proteinase I) has been shown to be localized in rat liverlysosomes. Lysosomes also contain a specific proteinase (Proteinase II) that catalyzes the release of a small peptide from the NH2-terminus of the native subunits. In fasted rabbits Proteinase II is released into the cytoplasm, together with Cathepsin A, but Proteinase I remains associated with the lysosomal fraction. Increased osmotic fragility of liver lysosomes in fasted rabbits has also been observed, but this increased fragility does not result in the release of Proteinase I. The appearance of Proteinase II in the cytoplasm may be due either to its selective release from the lysosomes, without release of Proteinase I, or its localization in a different lysosomal fraction. Changes in lysosomal structure induced by fasting may play a dual role in : 1) the mobilization of amino acids for gluconeogenesis and 2) the modulation of activity of gluconeogenic enzymes.

Acid Phosphatase↗
PubMed1976

Dual role of Zn2+ as inhibitor and activator of fructose 1,6-bisphosphatase of rat liver.

At neutral pH, Zn2+ is a potent and specific inhibitor of rat liver fructose 1,6-bisphosphatase (EC 3.1.3.11; D-fructose-1,6-bisphosphate 1-phosphohydrolase). Inhibition by Zn2+ is uncompetitive with respect to the activating cations Mg2+ and Mn2+, and the kinetic data suggest that the enzyme possesses a distinct high-affinity binding site for Zn2+, with Ki of approximately 0.3 muM. At higher concentrations (about 10(-5) M) Zn2+, and to a lesser extent Co2+, function as activating cations. Binding studies show that the enzyme binds two equivalents of Zn2+ per subunit; one equivalent is partially displaced by Mg2+ and is presumably bound to the site for activating cations. A second equivalent binds to the high-affinity site, presumably identical to the inhibitory site. The results suggest that Zn2+ functions as an allosteric regulator, and that the commonly observed activation of fructose 1,6-bisphosphatase at neutral pH by EDTA, histidine, and other chelators is due to removal of endogenous Zn2+ by these agents.

Animals↗
PubMed1976

The biochemistry of sugars.

In animal cells the utilization of sugars involves one or more of the three major pathways known to occur in these cells. The glycolytic (Embden-Meyerhof) pathway is quantitatively the most important, and this pathway, coupled to the citric acid cycle, serves as a major source of energy. Glycolysis and the citric acid cycle also provide most of the precursors for the synthesis of proteins, nucleic acids and lipids. The pentose phosphate, or hexose monophosphate oxidation, pathway is a major source of NADPH required for the conversion of carbohydrate to the more reduced lipids and proteins, and also furnishes the ribose and deoxyribose moieties of nucleotides and nucleic acids. The functions of the third pathway, the uronic acid pathway, are less well defined. It is the source of glucuronides for mucopolysaccharides and for detoxification mechanisms, and also in most animals, is the pathway for the synthesis of ascorbic acid. The uronic acid pathway may also function as a point of entry for glucuronides, pentoses, and pentitols.

Amino Acids↗