No abstract

Reaction Centers (RCs) from the photosynthetic bacterium Rhodopseudomonas sphaeroides were incorporated in planar bilayers made from monolayers derived from liposomes reconstituted with purified RCs. The photocurrents associated with the charge recombination process between the reduced primary quinone (QA-) and the oxidized bacteriochlorophyll donor (D+) were measured as a function of voltage (-150 mV less than V less than 150 mV) applied across the bilayer. When QA was the native ubiquinone (UQ) the charge recombination was voltage independent. However, when UQ was replaced by anthraquinone (AQ), the recombination time depended on the applied voltage V according to the relation tau = 8.5 X 10(-3) eV/0.175S. These results were explained by a simple model in which the charge recombination from UQ- proceeds directly to D+ while that from AQ occurs via a thermally activated intermediate state, D+I-QA, where I is the intermediate acceptor. The voltage dependence arises from an electric field induced change in the energy gap, delta G0, between the states D+I-QA and D+IQA-. This model is supported by the measured temperature dependence of the charge recombination time, which for RCs with AQ gave a value of delta G0 = 340 +/- 20 meV. In contrast, delta G0 for RCs with UQ as the primary acceptor, is sufficiently large (approximately 550 meV) so that even in the presence of the field, the direct pathway dominates. The voltage dependence shows that the electron transfer from I- to QA is electrogenic. From a quantitative analysis of the voltage dependence on the recombination rate it was concluded that the component of the distance between I and QA along the normal to the membrane is about one-seventh of the thickness of the membrane. This implies that the electron transfer from I to Q contributes at least one-seventh to the potential generated by the charge separation between D+ and QA-.

High-dose busulfan (Bu) is frequently used in preparative myeloablative conditioning (MAC) regimens for patients undergoing hematopoietic stem cell transplantation (HSCT). MAC and reduced-intensity conditioning (RIC) protocols for i.v. Bu infusion have been developed to achieve reliable systemic exposure while minimizing toxicity and treatment failure (relapse). The objectives of the present study were to (1) compare the pharmacokinetics (PK) of i.v. Bu in different dosing protocols, (2) compare intrasubject variability of Bu PK over repeated administrations; (3) examine the effect of concomitant administration of fludarabine on Bu PK, and (4) examine the effect of plasma concentrations of glutathione (GSH), the cosubstrate in Bu metabolism, on Bu clearance. We studied Bu PK twice in each of 46 HSCT patients (after the first and then after the middle dose of the treatment cycle) receiving one of 4 dosing protocols, 2 MAC (cumulative dose, 12.8 mg/kg) and 2 RIC (cumulative dose, 6.4 mg/kg), with daily doses administered either as an individual infusion (3.2 mg/kg) or as 4 infusions of 0.8 mg/kg each. Blood samples were obtained for 6-24 hours after dosing for measurement of Bu plasma concentrations. PK parameters were estimated using compartmental analyses. In a subgroup of patients (n = 14), GSH blood concentrations were determined before Bu administration. Dose- and weight-corrected Bu PK parameters (clearance, 0.173 ± 0.051 L/hour · kg; volume of distribution, 0.71 ± 0.17 L/kg; half-life time, 3.0 ± 0.7 hours) did not differ among treatment protocols (all P >.14) and remained stable between the first and mid-cycle doses. Fludarabine did not affect Bu PK. Blood GSH concentrations before Bu dosing were positively correlated with Bu clearance (adjusted R(2) = 0.45; P = .009). Our data indicate that Bu PK parameters are linear, stable, and predictable in different i.v. protocols and are unaffected by coadministration of fludarabine. Differences in whole blood GSH might contribute to variability in Bu clearance.

A quantitative analysis of the pathways leading to glycogen repletion in rats was conducted. [U‐13C]Glucose was administered intra‐intestinally into awake fasted animals. The distribution of glucose isotopomers derived from liver glycogen, liver extracts and plasma was performed by GC‐MS and 13C NMR. The potential gluconeogenic precursors for liver glycogen, lactate, alanine, glutamate and glutamine, were also analyzed. The amount of glycogen that is synthesized by the direct pathway was found to be 35%. The 13C enrichment of liver lactate, alanine and glucose is similar, indicating that they are the major precursors for liver glycogen synthesis via the indirect pathway. Our results demonstrate that after 24 h fasting, when glucose is supplied, gluconeogenesis from endogenous sources is not shut off.