Mechanism-based inactivation of bovine adrenal cytochromes P450 C-21 and P450 17 alpha by 17 beta-substituted steroids

A series of progesterone derivatives has been studied as potential inactivators of the bovine adrenocortical cytochromes P450, P450 17 alpha, and P450 C-21. Replacement of the 21-methyl group of progesterone with a difluoromethyl group resulted in a selective inactivator of P450 C-21 in a reconstituted system. The loss of 21-hydroxylase activity caused by this compound exhibits a number of characteristics of mechanism-based inactivation including NADPH dependence, pseudo-first-order kinetics, saturability, irreversibility, and protection by substrate. In addition to the difluoro compound, 21,21-dichloroprogesterone, the acetylenic compound pregn-4-en-20-yn-3-one, and the olefinic compound pregna-4,20-dien-3-one all inactivate P450 C-21. In contrast, the only compound to inactivate the rabbit adrenal progesterone 21-hydroxylase is 21,21-dichloroprogesterone. In binding studies, the 21,21-dihalo steroids produce a greater maximal type I spectral shift of P450 C-21 than the two 17 beta-unsaturated steroids. The dihalo compounds inactivate P450 C-21 by both heme destruction and protein modification as shown by significant decreases in residual 21-hydroxylase activity and spectrally detectable P450 after incubation with P450 C-21 in a reconstituted system. Liquid chromatographic and mass spectral analyses of the organic extracts from these incubations showed that 21-pregnenoic acid is a major metabolite of the dihalo compounds with a partition ratio of 5 nmol of acid produced/nmol of P450 C-21 inactivated. This supports the hypothesis that inactivation proceeds in part through an acyl halide intermediate. In contrast, the acetylenic compound pregn-4-en-20-yn-3-one inactivates P450 C-21 mainly by protein modification, producing an NADPH-dependent irreversible type I spectral shift. The stoichiometry of inactivation is approximately 1.5 nmol of compound bound/nmol of enzyme inactivated, indicating selective modification of the enzyme at or near the substrate binding site.     

Proposed follow up programme after curative resection for lower third oesophageal cancer

Cyclins are indispensable elements of the cell cycle and derangement of their function can lead to cancer formation. Recent studies have also revealed more mechanisms through which cyclins can express their oncogenic potential. This review focuses on the aberrant expression of G1/S cyclins and especially cyclin D and cyclin E; the pathways through which they lead to tumour formation and their involvement in different types of cancer. These elements indicate the mechanisms that could act as targets for cancer therapy.     

Laser-induced withdrawal test for electrophysiological recordings of nociception

CO₂ lasers are often used in pain research. However, the stimulation parameters of the CO₂ lasers, such as beam diameter, laser power, etc., used for these animal nociceptive studies vary across laboratories. The differences of the parameters usually make novices who want to reproduce the laser-evoked responses confused to follow. In this study, we quantitatively measured the laser-withdrawal reflex of the rat to ascertain the individual laser-withdrawal threshold and then found the thresholds were diverse among the rats we adopted. Furthermore, the optimal stimulation distance, the most conveniently modified stimulation parameter of CO₂ laser stimuli, was also determined. We suggest that laser-withdrawal tests should be applied before the electrophysiological recordings in order to verify the efficiency of the induced nociception and substantiate the operational parameters of the CO₂ laser device.