Kinetic and structural details of urease inactivation by thiuram disulphides.

This paper reports on the molecular details of the reactivity of urease, a nickel-dependent enzyme that catalyses the last step of organic nitrogen mineralization, with thiuram disulphides, a class of molecules known to inactivate the enzyme with high efficacy but for which the mechanism of action had not been yet established. IC50 values of tetramethylthiuram disulphide (TMTD or Thiram) and tetraethylthiuram disulphide (TETD or Disulfiram) in the low micromolar range were determined for plant and bacterial ureases. The X-ray crystal structure of Sporosarcina pasteurii urease inactivated by Thiram, determined at 1.68 Å resolution, revealed the presence of a covalent modification of the catalytically essential cysteine residue. This is located on the flexible flap that modulates the size of the active site channel and cavity. Formation of a Cys-S-S-C(S)-N(CH3)2 functionality responsible for enzyme inactivation was observed. Quantum-mechanical calculations carried out to rationalise the large reactivity of the active site cysteine support the view that a conserved histidine residue, adjacent to the cysteine in the active site flap, modulates the charge and electron density along the thiol SH bond by shifting electrons towards the sulphur atom and rendering the thiol proton more reactive. We speculate that this proton could be transferred to the nickel-coordinated urea amide group to yield a molecule of ammonia from the generated Curea-NH3+ functionality during catalysis.

Greater peripheral blood flow but less bleeding with propofol versus sevoflurane during spine surgery: a possible physiologic model?

STUDY DESIGN: Prospective, randomized, single blind. OBJECTIVE: To compare the effects of sevoflurane and propofol on lumbar-paraspinal-muscles regional blood flow, as well as bleeding when controlled hypotension is used. SUMMARY OF BACKGROUND DATA: Controlled hypotension is the technique of choice to reduce blood loss during spine surgery, but changes in blood flow occurring to lumbar paraspinal muscles during controlled hypotension with propofol and sevoflurane, as well as the entity of bleeding, are unknown. METHODS: Blood flow was assessed by means of a laser Doppler flowmeter during the prehypotensive and hypotensive (defined as a 15% reduction of baseline mean arterial pressure) period in 28 patients (aged 28-73 years, American Society of Anesthesiologists (ASA) I-II) undergoing lumbar spine surgery. Patients were randomized to receive either sevoflurane or propofol as main anesthetic agent to achieve hypotension. At the end of the surgery, blood loss was calculated and intraoperative bleeding (Visual Analogue Scale ranging from 0 to 100) was evaluated by the surgeon. RESULTS.: Peripheral Blood flow was significantly greater in the propofol group both before and during the hypotensive period (median values of 32.7 FU vs. 7.7 and 38.5 FU vs. 10.5, respectively). Despite this fact, blood loss and intraoperative bleeding were significantly reduced when propofol had been used (P < 0.05). CONCLUSION: Despite the greater blood flow when it is used, propofol causes less bleeding than sevoflurane during spine surgery and could be more indicated to produce hypotension during anesthesia. Moreover, it is possible to explain our findings hypothesizing a selective vasodilation of propofol (postcapillary, venous vasodilation), different from that of sevoflurane (precapillary, arteriolar vasodilation).