The fallacy of the P crit - are there more useful alternatives?

P _crit - generally defined as the P _O2  below which the animal can no longer maintain a stable rate of O₂ consumption (Ṁ _O2 ), such that Ṁ _O2  becomes dependent upon P _O2  - provides a single number into which a vast amount of experimental effort has been invested. Here, with specific reference to water-breathers, I argue that this focus on the P _crit is not useful for six reasons: (1) calculation of P _crit usually involves selective data editing; (2) the value of P _crit depends greatly on the way it is determined; (3) there is no good theoretical justification for the concept; (4) P _crit is not the transition point from aerobic to anaerobic metabolism, and it disguises what is really going on; (5) P _crit is not a reliable index of hypoxia tolerance; and (6) P _crit carries minimal information content. Preferable alternatives are loss of equilibrium (LOE) tests for hypoxia tolerance, and experimental description of full Ṁ _O2  versus P _O2  profiles accompanied by measurements of ventilation, lactate appearance and metabolic rate by calorimetry. If the goal is to assess the ability of the animal to regulate Ṁ _O2  from this profile in a mathematical fashion, promising, more informative alternatives to P _crit are the regulation index and Michaelis-Menten or sigmoidal allosteric analyses.

A new, sensitive ecto-5'-nucleotidase assay for compound screening

Ecto-5'-nucleotidase (eN) is a membrane-bound enzyme that hydrolyzes extracellular nucleoside-5'-monophosphates yielding the respective nucleoside and phosphate. Increased levels of eN expression have been observed in many cancer cells. By increasing extracellular adenosine concentrations, they contribute to their proliferative, angiogenic, metastatic, and immunosuppressive effects. Therefore, eN is of considerable interest as a novel drug target for the treatment of cancer as well as of inflammatory diseases. In this study, we developed, optimized, and applied a highly sensitive radiometric assay using [³H]adenosine-5'-monophosphate (AMP) as a substrate. The reaction product [³H]adenosine was separated from [³H]AMP by precipitation of the latter with lanthanum chloride and subsequent filtration through glass fiber filters. Conditions were optimized to reproducibly collect the [³H]adenosine-containing filtrate used for quantitative determination. Validation of the assay yielded a mean Z' factor of 0.73, which demonstrates its suitability for high-throughput screening. The new assay shows a limit of detection that is at least 30-fold lower than those of common colorimetric methods (e.g., optimized malachite green assay and capillary electrophoresis-based assay procedures), and it is also superior to a recently developed luciferase-based assay.