ON THE DETERMINATION OF ORNITHINE-CARBAMYL-TRANSFERASE ACTIVITY

Effect of UV-B (302 nm) irradiation on isolated rat hepatocytes

The present study aimed to evaluate the effect of UV-B irradiation on the functional integrity, and the metabolic and detoxifying capacity of isolated rat hepatocytes. Isolated rat hepatocytes were irradiated in various doses (400 Jm-2, 600 Jm-2, 800 Jm-2 and 1000 Jm-2). The cells were assayed for total lactate dehydrogenase, Na(+)-K(+)-ATPase, ornithine carbamyltransferase activity (OCT) and urea production capacity. Lactate dehydrogenase and Na(+)-K(+)-ATPase activity were significantly decreased in all four irradiated groups (P < 0.001), whereas viability, OCT and urea production capacity showed no alterations.

Urea cycle enzymes in retina, ciliary body-iris, lens and senile cataracts

Carbamylation of lens proteins induced conformational changes and may play a role in the development of cataracts in uremic patients. Thus, the activities of the urea cycle enzymes: carbamyl phosphate synthetase I, ornithine transcarbamylase, argininosuccinate synthetase, argininosuccinase and arginase, were determined in lens, retina and ciliary body-iris of calf and rabbit. No ornithine transcarbamylase activity was found in ciliary body-iris, lens and retina of calf and rabbit whereas carbamyl phosphate synthetase I, argininosuccinate synthetase, argininosuccinase and arginase activities in calf lens were 5.02 +/- 0.21, 9.50 +/- 0.29, 9.17 +/- 0.16 and 6.32 +/- 0.19 [mumol (g protein)-1 hr-1], respectively. Except arginase, the activities of carbamyl phosphate synthetase I, argininosuccinate synthetase and argininosuccinase in lens were 30-50% of the values in retina or ciliary body-iris. The Km for each of the substrates was obtained for argininosuccinate synthetase, argininosuccinase and arginase of calf lens. Activities of carbamyl phosphate synthetase I, argininosuccinate synthetase, argininosuccinase and arginase in clear human lenses, aged 67-87 years, were 0.11 +/- 0.01, 0.67 +/- 0.01, 0.20 +/- 0.01 and 0.58 +/- 0.03 (mumol lens-1 hr-1), respectively. Two-fold increase in the activity of arginase was found in senile cataracts, but all other enzymes had 36-87% decreases in activities. It is likely that the rise in arginase activity in cataracts could facilitate polyamine synthesis through ornithine and ornithine decarboxylase and additional formation of cyanate, a carbamylating compound, both of which have been implicated in cataract formation. Further, decreased activities of argininosuccinate synthetase and argininosuccinase together with increased arginase activity could lead to the depletion of arginine in senile cataracts.

A direct method for the estimation of ornithine carbamoyltransferase activity in serum

Ornithine carbamoyltransferase (EC 2.1.3.3) activity was estimated by determining the amount of citrulline produced. The citrulline was colorimetrically measured by using a diacetylmonoxime-thiosemicarbazide reaction without deproteinization. The color complex which had the maximum absorbance at 515 nm was stable at room temperature in daylight for at least several hours. The proposed method was superior to other methods already reported in sensitivity, stability and simplicity.

The pathogenesis of lupinosis in the rat

The morphological, histochemical and biochemical changes in the liver of rats challenged with a single intraperitoneal injection of the toxin of lupinosis were investigated. The resulting injury is biphasic. The initial non-specific phase lasts for a few days, affects mainly the centrilobular region and is characterised by mitochondrial swelling, lipid accummulation and autophagosome formation. In addition, the hepatic concentrations of OCT, LDH, SDH, and AST fall while that for ALP increase. This is accompanied by a rise in the serum concentration of many of these enzymes. The second phase lasts longer and is characterised by phenomena indicative of abnormalities in hepatocytic division. Normal and abnormal metaphase as well as multinucleated hepatocytes falls. The possibility that the toxin of lupinosis interferes with microtubular units is considered.