Kinetics of the 5′-nucleotidase and the adenosine deaminase in subcellular fractions of rat brain

Suspensions of rat brain microsomes, synaptosomes, and synaptic vesicles were able to convert adenosine to inosine by means of adenosine deaminase. Isosbestic points of this transformation, at 222, 250 and 281 nm, remained unchanged with time-course. This fact suggests that adenosine deaminase (ADA, E.C. 3.5.4.4) is located on the surface of the vesicles whereas purine nucleoside phosphorylase (PNP, E.C. 2.1.2.4) is located inside the vesicles. Kinetic parameters of the particulate 5-nucleotidase (5N, E.C. 3.1.3.5) and adenosine deaminase were analogous to those of the cytosolic enzymes. These results suggest that soluble and particulate enzymes represent different pools of the same molecular species.     

Further characterization of adenosine transport in renal brush-border membranes

Adenosine transport has been further characterized in rat renal brush-border membranes (BBM). The uptake shows two components, one sodium-independent and one sodium-dependent. Both components reflect, at least partly, translocation via a carrier mechanism, since the presence of adenosine inside the vesicles stimulates adenosine uptake in the presence as well as in the absence of sodium outside the vesicles. The sodium-dependent component is saturable (Km adenosine = 2.9 microM, Vmax = 142 pmol/min per mg protein) and is abolished at low temperatures. The sodium-independent uptake has apparently two components: one saturable (Km = 4-10 microM, Vmax = 174 pmol/min per mg protein) and one non-saturable (Vmax = 3.4 pmol/min per mg protein, Km greater than 2000 microM). Inosine, guanosine, 2-chloroadenosine and 2'-deoxyadenosine inhibit the sodium-dependent and -independent transport, as shown by trans-stimulation experiments, probably because of translocation via the respective transporter. Uridine and dipyridamole inhibited only the sodium-dependent uptake. Other analogs of adenosine showed no inhibition. The kinetic parameters of the inhibitors of the sodium-dependent component were further investigated. Inosine was the most potent inhibitor with a Ki (1.9 microM) less than the Km of adenosine. This suggests a physiological role for the BBM ecto-adenosine deaminase (enzyme which extracellularly converts adenosine to inosine), balancing the amount of nucleoside taken up as adenosine or inosine by the renal proximal tubule cell.     

Making and working with hydrogen sulfide: The chemistry and generation of hydrogen sulfide in vitro and its measurement in vivo: A review

Hydrogen sulfide is rapidly emerging as an important vasoactive mediator formed in health and disease. Its biological action is centered on its reactivity with heme-proteins and its ability to activate K_ATP channels. Hydrogen sulfide is a signalling molecule of the inflammatory and nervous systems, and in particular the cardiovascular system where it regulates vascular tone, cardiac work, and exerts cardioprotection.This has led to an explosion of papers in which the role of hydrogen sulfide generated in vitro has been used to stimulate biological responses, and where a variety of methods have been used to measure the concentration of this compound in biological fluids. Understanding the chemistry and the inherent problems in the analytical techniques used to measure hydrogen sulfide concentrations is critical to our expanding knowledge on the biology of hydrogen sulfide. In this brief review we will cover the chemistry of hydrogen sulfide, including sources of hydrogen sulfide, its speciation at physiological pH, the susceptibility of sulfide to aerobic oxidation, and the methods used to measure hydrogen sulfide concentrations in solution, including biological fluids. We also give a brief overview of knockout animals and inhibition of the enzymes involved in the formation of hydrogen sulfide in vivo.     

Green tea phenolics inhibit butyrate-induced differentiation of colon cancer cells by interacting with monocarboxylate transporter 1

Diet has a significant impact on colorectal cancer and both dietary fiber and plant-derived compounds have been independently shown to be inversely related to colon cancer risk. Butyrate (NaB), one of the principal products of dietary fiber fermentation, induces differentiation of colon cancer cell lines by inhibiting histone deacetylases (HDACs). On the other hand, (?)-epicatechin (EC) and (?)-epigallocatechin gallate (EGCG), two abundant phenolic compounds of green tea, have been shown to exhibit antitumoral properties. In this study we used colon cancer cell lines to study the cellular and molecular events that take place during co-treatment with NaB, EC and EGCG. We found that (i) polyphenols EC and EGCG fail to induce differentiation of colon adenocarcinoma cell lines; (ii) polyphenols EC and EGCG reduce NaB-induced differentiation; (iii) the effect of the polyphenols is specific for NaB, since differentiation induced by other agents, such as trichostatin A (TSA), was unaltered upon EC and EGCG treatment, and (iv) is independent of the HDAC inhibitory activity of NaB. Also, (v) polyphenols partially reduce cellular NaB; and (vi) on a molecular level, reduction of cellular NaB uptake by polyphenols is achieved by impairing the capacity of NaB to relocalize its own transporter (monocarboxylate transporter 1, MCT1) in the plasma membrane. Our findings suggest that beneficial effects of NaB on colorectal cancer may be reduced by green tea phenolic supplementation. This valuable information should be of assistance in choosing a rational design for more effective diet-driven therapeutic interventions in the prevention or treatment of colorectal cancer.