Quantitative surface analysis of Cardiothane 51 by FT-IR-ATR spectroscopy

Surface analysis has been carried out for Cardiothane 51 samples as commercially available balloon pumps and cast films prepared from the solution. Silicone contents on surfaces have been quantitatively determined by FT-IR-ATR spectroscopy coupled with a subtraction technique. The contents on the air-facing surface vary from 0.7 to 5.3% among six balloon pumps examined; three of them contain less silicone on the air-facing surface than on the substrate-facing surface, whereas just the opposite is true for one, and the other two have nearly and quite an equal content of it between the two surfaces. Silicone contents on surfaces of cast films vary also from a sample to another even when sample films were prepared from the same Cardiothane solution, indicating difficulty in quality control of Cardiothane surfaces. An appreciable fraction of silicone was found not to be covalently bonded to polyurethane in Cardiothane 51 for the cast film samples.

D-glucosaminate dehydratase: spectrometric properties of the enzyme-bound pyridoxal 5'-phosphate

The holoenzyme of D-glucosaminate dehydratase [EC 4.2.1.26] from Agrobacterium radiobacter showed absorption peaks at 280 and 415 nm with a shoulder in the region of 320 to 330 nm. The treatment of the enzyme with hydroxylamine followed by dialysis led to disappearance of both the absorption peak at 415 nm and the shoulder, giving the apoenzyme. The fluorescence excitation maximum of the holoenzyme was at 320 nm with a shoulder at 420 nm (emission at 510 nm), and the emission maxima were at 420 nm with a shoulder at 370 nm (excitation at 320 nm) and at 510 nm (excitation at 420 nm). The holoenzyme showed a negative circular dichroic band at 418 nm and a positive shoulder at around 320 nm. Reduction of the holoenzyme with sodium borohydride caused a loss of the absorption peak at 415 nm with a concomitant increase of 325 nm absorbance and an irreversible loss of the activity. The occurrence of epsilon-N-pyridoxyllysine in the acid hydrolysate of the reduced enzyme showed that D-glucosaminate dehydratase contains a catalytically essential lysine residue whose epsilon-amino group binds the 4-formyl group of pyridoxal 5'-phosphate to form a Schiff base. The plots of absorption of the apoenzyme against the amount of pyridoxal 5'-phosphate added showed that four and two molar equivalents of the cofactor bind to the apoenzyme and subunit, respectively. The biphasic nature of the spectrometric titration curve of the apoenzyme with pyridoxal 5'-phosphate and the two Km values obtained for the cofactor suggest the occurrence of two distinct types of binding sites for pyridoxal 5'-phosphate in the enzyme.

Stereochemistry of an alpha, beta-elimination reaction by D-glucosaminate dehydratase

The proton NMR analysis of D-glucosaminate dehydratase reaction in D2O revealed the incorporation of a deuterium atom at C-3 carbon of the product, 2-keto-3-deoxy-D-gluconate. Based on the chemical shift of C-3 proton of the product and the coupling constant characteristic for the C-3 and C-4 axial-axial coupling in the 2C5 pyranose conformation, the deuterium is in the pro-S position. Thus, the dehydration of D-glucosaminate by the enzyme proceeds in a retention mode at C-3 carbon. Kinetic parameters show that the rate-determining step is the abstraction of alpha-proton from the substrate.

D-glucosaminate dehydratase from Agrobacterium radiobacter. Physicochemical and enzymological properties

The bacterial distribution of D-glucosaminate dehydratase [EC 4.2.1.26] was investigated and Agrobacterium radiobacter (IAM 1526) was found to have the highest enzyme activity. The enzyme was formed inducibly in a glycerol-urea medium by D-glucosamine, D-galactosamine, and D-glucosamine, but not by D-mannosamine. The enzyme purified from the cells grown in the glucosamine-glycerol-urea medium was shown to be homogeneous by ultracentrifugation. The molecular weight was determined to be about 66,000 by the sedimentation equilibrium method, and 72,800 by the gel permeation chromatography low-angle light scattering method. The pH optimum is 8.3-9.0. The enzyme catalyzed the dehydration of D-glucosaminate (relative activity: 100, Km: 2.8 mM), D-galactosaminate (31.5, 5.0 mM), D-mannosaminate (17.5, 29 mM), D-threonine (5.1, 4.8 mM), D-serine (3.2, 0.026 mM), and L-serine (1.1, ND), but not L-threonine. The reverse reaction does not occur. The enzyme is inhibited by typical inhibitors of pyridoxal 5'-phosphate enzymes, such as L'penicillamine, and also by carbonyl reagents, thiol reagents, divalent metals, and several D-amino acids and D-amino sugars.