RESUMEN

This study evaluated the indications, surgical techniques, and outcomes of allograft pancreatectomy based on a single center experience. Between 2003 and 2013, 47 patients developed pancreas allograft failure, excluding mortality with a functioning pancreas allograft. Early graft loss (within 14 days) occurred in 16, and late graft loss in 31. All patients with early graft loss eventually required allograft pancreatectomy. Nineteen of 31 patients (61%) with late graft loss underwent allograft pancreatectomy. The main indication for early allograft pancreatectomy included vascular thrombosis with or without severe pancreatitis, whereas one recipient required urgent allograft pancreatectomy for gastrointestinal hemorrhage secondary to an arterioenteric fistula. In cases of late allograft pancreatectomy, graft failure with clinical symptoms such as abdominal discomfort, pain, and nausea were the main indications (13/19 [68%]), simultaneous retransplantation without clinical symptoms in 3 (16%), and vascular catastrophes including pseudoaneurysm and enteric arterial fistula in 3 (16%). Postoperative morbidity included one case each of pulmonary embolism leading to mortality, formation of pseudoaneurysm requiring placement of covered stent, and postoperative bleeding requiring relaparotomy eventually leading to femoro-femoral bypass surgery 2 years after allograftectomy. Allograft pancreatectomy can be performed safely, does not preclude subsequent retransplantation, and may be lifesaving in certain instances.

Asunto(s)

RESUMEN

Candidacy for retransplantation after allograft loss due to BK virus-associated nephropathy (BKVN) with or without allograft nephrectomy is controversial. This report describes 2 renal transplant recipients who lost their grafts to BKVN and subsequently underwent simultaneous kidney and pancreas transplantation with allograft nephrectomy.

Asunto(s)

RESUMEN

Early pancreas allograft failure most commonly results from thrombosis and requires immediate allograft pancreatectomy. Optimal timing for retransplantation remains undefined. Immediate retransplantation facilitates reuse of the same anatomic site before extensive adhesions have formed. Some studies suggest that early retransplantation is associated with a higher incidence of graft loss. This study is a retrospective review of immediate pancreas retransplants performed at a single center. All cases of pancreas allograft loss within 2 weeks were examined. Of 228 pancreas transplants, 12 grafts were lost within 2 weeks of surgery. Eleven of these underwent allograft pancreatectomy for thrombosis. One suffered anoxic brain injury and was not a retransplantation candidate, one was retransplanted at 3.5 months and nine patients underwent retransplantation 1-16 days following the original transplant. Of the nine early retransplants, one pancreas was lost to heparin-induced thrombocytopenia, one recipient died with function at 2.9 years and the other grafts continue to function at 76-1137 days (mean 572 days). One-year graft survival for early retransplantation was 89% compared to 91% for all pancreas transplants at our center. Immediate retransplantation following pancreatic graft thrombosis restores durable allograft function with outcomes comparable to first-time pancreas transplantation.

Asunto(s)

Asunto(s)

RESUMEN

Penicillin G acylase catalysed the hydrolysis of 4-nitrophenyl acetate with a kcat of 0.8 s-1 and a Km of 10 microM at pH 7.5 and 20 degreesC. Results from stopped-flow experiments fitted a dissociation constant of 0.16 mM for the Michaelis complex, formation of an acetyl enzyme with a rate constant of 32 s-1 and a subsequent deacylation step with a rate constant of 0.81 s-1. Non-linear Van't Hoff and Arrhenius plots for these parameters, measured at pH 7.5, may be partly explained by a conformational transition affecting catalytic groups, but a linear Arrhenius plot for the ratio of the rate constant for acylation relative to KS was consistent with energy-compensation between the binding of the substrate and catalysis of the formation of the transition state. At 20 degreesC, the pH-dependence of kcat was similar to that of kcat/Km, indicating that formation of the acyl-enzyme did not affect the pKa values (6.5 and 9.0) of an acidic and basic group in the active enzyme. The heats of ionization deduced from values of pKa for kcat, which measures the rate of deacylation, are consistent with alpha-amino and guanidinium groups whose pKa values are decreased in a non-polar environment. It is proposed that, for catalytic activity, the alpha-amino group of the catalytic SerB1 and the guanidinium group of ArgB263 are required in neutral and protonated states respectively.

Asunto(s)

RESUMEN

Colicins A, B, and N form a family of membrane pore-forming toxins with > 50% sequence identity in their toxic C-terminal domains. The colicin A C-terminal domain has been shown to insert into model membranes via an acidic molten-globule insertion intermediate, and thus this family provides a means to compare acid unfolding of related proteins. Unlike the domains of colicins A and B which are acidic, that of colicin N is very basic with fewer Asp and Glu residues. If surface positive charge density is the crucial factor in acidic molten globule formation, colicin N should begin to unfold at higher pH values than colicins A or B. However, comparison of their CD spectra reveals that colicins A and B both form acidic molten globules but colicin N does not. None of the proteins forms a denaturant-induced molten globule at neutral pH where the proteins exhibit very similar stabilities. The acidic unfolding cannot therefore be due to excess positive surface charge and may be caused by a subset of acidic residues as has been predicted for myoglobin. The difference between the colicins is confirmed by their in vivo membrane insertion, with colicins A and B inserting much faster than colicin N. Stopped-flow circular dichroism measurements of colicin A insertion into vesicles confirmed that a molten globule insertion intermediate occurs at the membrane surface.

Asunto(s)

RESUMEN

The kinetics of release of 4-nitrophenol were followed by stopped-flow spectrophotometry with two 4-nitrophenyl ester substrates of penicillin G acylase from Kluyvera citrophila. With the ester of acetic acid, but not of propionic acid, there was a pre-steady-state exponential phase, the kinetics of which were inhibited by phenylacetic acid (a product of hydrolysis of specific substrates) to the extent predicted from Ki values. This was interpreted as deriving from rapid formation (73 mM-1.s-1) and slow hydrolysis (0.76 s-1) of an acetyl derivative of the side chain of the catalytic-centre residue Ser-290. With the mutant F360V, which differs from the wild-type enzyme in its ability to hydrolyse adipyl-L-leucine and has a kcat for 4-nitrophenyl acetate one-twentieth that of the wild-type enzyme, the corresponding values for the rates of formation and hydrolysis of the acetyl-enzyme were 11.1 mM-1.s-1 and 0.051 s-1 respectively. The ratio of these rate constants was three times that for the wild-type enzyme, suggesting that the mutant is less impaired in the rate of formation of an acetyl-enzyme than in its subsequent hydrolysis.

Asunto(s)

RESUMEN

In contrast with the general thought that penicillin G acylases (PGAs) were only able to hydrolyse amides or esters of higly hydrophobic acids, we have demonstrated that the PGA from Kluyvera citrophila catalysed the hydrolysis of 4-nitrophenyl esters of acetic, propionic, butyric and valeric acids. Values of kcat. and kcat./Km were greatest for the first compound and less than values for benzylpenicillin by factors of 30 and 7, respectively. 4-Nitrophenyl acetate was hydrolysed faster than 2-nitrophenyl acetate but slower than phenyl acetate. The pH dependence of the reaction was similar to that of benzylpenicillin. Several experiments showed that hydrolysis of 4-nitrophenyl acetate was not catalysed by contaminating esterase activity. The implications for the structural basis of substrate binding are discussed. These substrates open, for the first time, a way to investigate the kinetic parameters of PGA at the presteady-state and provides a new perspective about the role of PGA in nature.

Asunto(s)