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J Tipper

Publications and source records attributed to J Tipper.

5 recordsLinked to original sources
PubMed2000

Towards a fibrous composite with dynamically controlled stiffness: lessons from echinoderms.

Sea urchins and sea cucumbers, like other echinoderms, control the tensile properties of their connective tissues by regulating stress transfer between collagen fibrils. The collagen fibrils are spindle-shaped and up to 1 mm long with a constant aspect ratio of approx. 2000. They are organized into a tissue by an elastomeric network of fibrillin microfibrils. Interactions between the fibrils are regulated by soluble macromolecules that are secreted by local, neurally controlled, effector cells. We are characterizing the non-linear viscoelastic properties of sea cucumber dermis under different conditions, as well as the structures, molecules and molecular interactions that determine its properties. In addition, we are developing reagents that will bind covalently to fibril surfaces and reversibly form cross-links with other reagents, resulting in a chemically controlled stress-transfer capacity. The information being developed will lead to the design and construction of a synthetic analogue composed of fibres in an elastomeric matrix that contains photo- or electro-sensitive reagents that reversibly form interfibrillar cross-links.

Animals↗
PubMed1992

"7-tetrahydrobiopterin," a naturally occurring analogue of tetrahydrobiopterin, is a cofactor for and a potential inhibitor of the aromatic amino acid hydroxylases.

The ability of 2-amino-4-hydroxy-7-[dihydroxylpropyl-(L-erythro)-5,6,7,8-tetrahyd ropterin] ("7-tetrahydrobiopterin" or 7-BH4) to substitute for the natural cofactor tetrahydrobiopterin (BH4) has been studied in vitro in the reactions of the three mammalian aromatic amino acid hydroxylases. With rat liver phenylalanine hydroxylase, the apparent Km for 7-BH4 is 160 microM, a value that is approximately 60-fold greater than that for the natural cofactor. In contrast, the hydroxylase reaction is severely inhibited by as little as 1 microM 7-BH4 when assayed in the presence of physiological concentrations of BH4. This inhibition can be overcome either by an increase in the concentration of BH4 or a decrease in the concentration of phenylalanine. With both rat brain tryptophan hydroxylase and rat pheochromocytoma tyrosine hydroxylase, the Km value for 7-BH4 is about one order of magnitude greater than the Km for BH4. Accordingly, 7-BH4 is a poor competitive inhibitor of both tryptophan and tyrosine hydroxylase. Thus, our results suggest that the observed hyperphenylalaninemia in patients who excrete 7-BH4 in their urine may arise directly from the inhibition of phenylalanine hydroxylase by low levels of this pterin. On the other hand, it is less likely that low levels of 7-BH4 would affect the activity of tyrosine or tryptophan hydroxylase in vivo.

Animals↗
PubMed1992

Phenylalanine-induced phosphorylation and activation of rat hepatic phenylalanine hydroxylase in vivo.

Rats were given intraperitoneal injections of 2 mCi of carrier-free 32Pi and substances known to activate liver phenylalanine hydroxylase. After 30 min, these animals were anesthetized and their livers removed for analysis of enzyme activity, 32Pi incorporation into immunoprecipitated phenylalanine hydroxylase and [gamma-32P]ATP specific activity. Following glucagon treatment, rat liver phenylalanine hydroxylase activity was stimulated more than 6-fold when assayed in the presence of the natural cofactor, tetrahydrobiopterin (BH4). Glucagon injection also resulted in an incorporation of 0.41 mol of 32Pi/mol of hydroxylase subunit (approximately 50,000 Da). In vivo stimulation of phenylalanine hydroxylase activity and 32Pi incorporation by glucagon had been previously observed in this laboratory (Donlon, J., and Kaufman, S. (1978) J. Biol. Chem. 253, 6657-6659). However, we show for the first time in the present study that in vivo treatment with phenylalanine alone results in a 4-fold increase in the BH4-dependent activity of phenylalanine hydroxylase concomitant with a significant incorporation of phosphate into phenylalanine hydroxylase (0.51 mol of 32Pi/mol of hydroxylase subunit). It is further demonstrated in vivo that the combined treatment with phenylalanine and glucagon results in a greater than 10-fold stimulation of BH4-dependent activity and the greatest level of 32Pi incorporation (0.75 mol of 32Pi/mol of hydroxylase subunit). Phenylalanine did not produce an elevation in plasma glucagon in these animals. A model is, thereby, proposed with respect to the ligand binding effects of phenylalanine on the state of phosphorylation and activation of phenylalanine hydroxylase. The significance of these regulatory roles are considered in light of the probable physiological environment of the enzyme.

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PubMed1986

Interaction of the 56,000-dalton phosphoprotein phosphatase from reticulocytes with regulin and inhibitor 2.

The interaction of divalent metal ions with a homogeneous 56,000-dalton phosphoprotein phosphatase isolated from rabbit reticulocytes was studied. The effects of the ions on enzymatic activity and on fluorescence from a 3-(4-maleimidylphenyl)-4-methyl-7-(diethylamino)coumarin derivative of the protein were compared. Enzymatic activity is dependent on Mn2+. The apparent association constant for Mn2+ is about 0.5 mM-1 as judged from enzymatic activity and from changes in fluorescence caused by binding of the metal ion; Ca2+ and Mg2+ do not affect enzymatic activity and appear not to bind tightly to the enzyme; however, Co2+, Fe2+, and Zn2+ bind to the protein and inhibit the Mn2+-activated enzyme. The 56,000-dalton phosphoprotein phosphatase was found to interact with regulin, a spectrin-associated protein also isolated from reticulocytes, and with skeletal muscle phosphatase inhibitor 2. The interaction was followed by changes in the enzymatic activity and by quenching of fluorescence from the coumarin derivative of the phosphatase. Homogeneous regulin (Mr approximately 230,000) increases the activity of the enzyme severalfold; this stimulation is Mn2+-dependent. Inhibitor 2 decreases enzyme activity but only if the two proteins are preincubated in the absence of Mn2+. Comparable differences in the effect of Mn2+ were also observed in parallel experiments in which changes in fluorescence from the coumarin-labeled 56,000-dalton phosphatase were measured. In these experiments, it was shown that Mn2+ enhances the interaction between regulin and the 56,000-dalton phosphatase, but inhibits the interaction between the phosphatase and inhibitor 2.

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