In response to global ischemia, tissue xanthine dehydrogenase was converted to xanthine oxidase in all tissues with half-times of conversion at 370C of -3.6, 6, 7, and 14 h for the liver, kidney, heart, and lung, respectively. The time course of enzyme conversion at 40C was greatly extended with half-conversion times of 6, 5, 5, and 6 d for the respective tissues. Increases in xanthine oxidase activity were accompanied by the appearance of a distinct new protein species with greater electrophoretic mobility. The oxidase from ischemic rat liver was purified 781-fold and found to migrate with a higher mobility on native gels than the purified native dehydrogenase. Sodium dodecyl sulfate profiles revealed the presence of a single major band of 137 kD for the native dehydrogenase, whereas the oxidase had been partially cleaved generating polypeptides of 127, 91, and 57 kD.Polypeptide patterns for the oxidase resemble those seen following limited in vitro proteolysis of the native dehydrogenase supporting a proteolytic mechanism for the conversion of xanthine dehydrogenase to oxidase in ischemic rat liver.

Previous studies have proposed and supported a role for the proteolytic, irreversible conversion of xanthine dehydrogenase to xanthine oxidase (XO) in postischemic injury in a wide variety of organs. A second mechanism of conversion, due to sulfhydryl modification and reversible with dithiothreitol (DTT), is potentially important but has not been well investigated. In this study rat liver and kidney were found to produce significant amounts of DTT-reversible XO during normothermic global ischemia. Formation of reversible XO precedes that of irreversible XO by approximately 0.5 h with a strong correlation (r = 0.92) existing between the rate of irreversible XO formation and the concentration of reversible XO. The formation of reversible XO is preceded by a depletion of glutathione with concentrations of glutathione during ischemia correlating (r = 0.85) with the observed concentration of reversible XO. While a large increase in the extent of liver damage occurs concurrently with conversion in an in vivo liver model of liver ischemia, an ischemia-reperfusion regimen (1 h of ischemia plus 0.5 h of reperfusion) that resulted in no conversion caused significant elevations in serum glutamic pyruvic transaminase and serum glutamic-oxaloacetic transaminase. Rats depleted of XO by tungsten dieting release 65% less enzyme after the same insult, suggesting that endogenous XO may also participate in the damage process independent of any conversion.

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e d i a t r i c s , Providence, Rhode I s l a n d , and Genetics I n s t i t u t e , Cambridge, Massachusetts EP regulates erythropoiesis i n t h e f e t u s (F) , newborn (N), and juvenile (J). Delineating the developmental changes t h a t occur i n EP k i n e t i c s would be important i n explaining the possible r o l e t h a t EP plays i n the pathophysiology of anemia and polycythemia. W e investigated t h e differences i n EP k i n e t i c s i n the F, N, and J by infusing 3 5~ l a b e l l e d human Ep i n t o 5 F (126 day), 6 N (7-8 day), and 4 J (270 day) sheep. After bolus infusion, timed blood samples were obtained and the plasma was p r e c i p i t a t e d with 20% t r i c h l o r o a c e t i c a c i d. Plasma h a l f l i f e (Tb), volume of d i s t r i b u t i o n (Vd) and EP clearance (Cep) were determined from t h e b e t a decay curves of I n EP vs time : Group Tk (hours) .Vd (ml-kg-l) cep (ml.kg-lsmin-J-) Fetus (F) *16.3 f 4.7 516 f 182 0.36 f 0.05 Newborn (N) 11.2 f 3.6