The missing link in the eukaryotic ribosome cycle

A truly groundbreaking paper by Pisarev et al. (2007), recently published in Cell, has solved a wide gap in our knowledge of eukaryotic mRNA translation, by elucidating how ribosomes are released from mRNA after the termination step.     

From the urea cycle to autophagy: Alfred J. Meijer

Now that many of the components of the autophagy machinery have been identified, in particular the autophagy-related (Atg) proteins, increasing focus is being directed toward the role of autophagy in health and disease. Accordingly, it is of ever-greater importance to understand the central role of autophagy in cellular metabolism, a point with which many people will likely agree. However, in our rush to understand autophagy’s function in metabolism, we tend to overlook the role of metabolism in regulating autophagy, even though substantial work has been done on this topic. One of the pioneers in this area is Alfred “Fred” J. Meijer.     

From the urea cycle to autophagy: Alfred J. Meijer

Now that many of the components of the autophagy machinery have been identified, in particular the autophagy-related (Atg) proteins, increasing focus is being directed toward the role of autophagy in health and disease. Accordingly, it is of ever-greater importance to understand the central role of autophagy in cellular metabolism, a point with which many people will likely agree. However, in our rush to understand autophagy's function in metabolism, we tend to overlook the role of metabolism in regulating autophagy, even though substantial work has been done on this topic. One of the pioneers in this area is Alfred "Fred" J. Meijer.     

Treatment of urea cycle disorders

Recent advances in the treatment of inborn errors of urea synthesis have significantly decreased mortality. Treatment has included combining a high-quality low-protein diet with supplements of deficient metabolites and stimulation of alternate pathways of waste nitrogen excretion. Long-term alternate pathway therapy, using sodium benzoate and sodium phenylacetate, has generally been unassociated with signs of toxicity. However, acute intoxications have simulated hyperammonemic crises. Neurologic outcome appears to be primarily a function of duration of neonatal hyperammonemic coma, although ongoing accumulation of urea cycle intermediates may also play a role. Early recognition and treatment are critical if a good outcome is to be possible.     

Diagnosis of urea cycle disorders

Hyperammonemia in pediatrics can be due to a number of causes (defects of urea cycle enzymes or transport of its metabolites, organic acidurias, acyl-CoA dehydrogenase or carnitine deficiency, liver bypass or nonspecific insufficiency) requiring differentiated rapid treatment for a satisfactory prognosis. The specific diagnosis cannot be established by clinical means. Thus the work-up rests on biochemical analyses. The methods used are detailed and their interpretation discussed. An algorithm for the interpretation of the data which can easily be computerized is presented. The procedure has proven practicable in 126 patients with urea cycle disorders.     

Urea amidolyase of Candida utilis. Characterization of the urea cleavage reactions.

Evidence is presented that the enzymes catalyzing the three reactions involved in urea cleavage in Candida utilis, biotin carboxylation, urea carboxylation, and allophanate hydrolysis occur as a complex of enzymes. The allophanate-hydrolyzing activity could not be separated from the urea-cleaving activity using common methods of protein purification. Further, urea cleavage and allophanate hydrolysis activities are induced coordinately in cells grown on various nitrogen sources. The reactions involved in urea cleavage can be distinguished from one another on the basis of their sensitivities to (a) heat, (b) pH, and (c) chemical inhibitors. Evidence is presented for the product of the first reaction in urea cleavage, biotin carboxylation. Production of carboxylated enzyme is ATP dependent and avidin sensitive. Carboxylated enzyme is not observed in the presence of 1 mM urea.     

The urea cycle in the liver of arthritic rats

The urea cycle in the liver of adjuvant-induced arthritic rats was investigated using the isolated perfused liver. Urea production in livers from arthritic rats was decreased during substrate-free perfusion and also in the presence of the following substrates: alanine, alanine + ornithine, ammonia, ammonia + lactate, ammonia + pyruvate and glutamine but increased when arginine and citrulline + aspartate were the substrates. No differences were found with ammonia + aspartate, ammonia + aspartate + glutamate, aspartate, aspartate + glutamate and citrulline. Ammonia consumption was smaller in the arthritic condition when the substance was infused together with lactate or pyruvate but higher when the substance was simultaneously infused with aspartate or aspartate + glutamate. Glucose production tended to correlate with the smaller or higher rates of urea synthesis. Blood urea was higher in arthritic rats (+25.6%), but blood ammonia was lower (–32.2%). Critical for the synthesis of urea from various substrates in arthritic rats seems to be the availability of aspartate, whose production in the liver is probably limited by both the reduced gluconeogenesis and aminotransferase activities. This is indicated by urea synthesis which was never inferior in the arthritic condition when aspartate was exogenously supplied, being even higher when both aspartate and citrulline were simultaneously present. Possibly, the liver of arthritic rats has a different substrate supply of nitrogenous compounds. This could be in the form of different concentrations of aspartate or other aminoacids such as citrulline or arginine (from the kidneys) which allow higher rates of hepatic ureogenesis.     

Connecting links between the urea cycle and the TCA cycle: a tutorial exercise

Some interesting points arose in a discussion on the links between the urea cycle and the TCA cycle. That aspartate is regenerated from fumarate is well known. One of the prime precursors of urea, the bicarbonate ion, is also formed from the CO₂, which is generated by the TCA cycle. The flux of acetyl CoA through the TCA cycle can indirectly affect the urea cycle by altering the levels of N-acetyl glutamate.