We previously constructed plasmids for synthesis of glutathione-peroxidase (GPx) mutants in an Escherichia coli expression system. In these recombinant proteins either cysteine ([CysIGPx mutant) or serine ([SerIGPx mutant) were present in place of the active-site selenocysteine (SeCys) of the natural enzyme. We have now investigated Selenium-glutathione peroxidase (SeGPx) catalyzes the reduction of hydrogen peroxide and that of most organic hydroperoxides (ROOH), using reduced glutathione (GSH) as reducing equivalent (Mills, 1957 ;Little and O'Brien. 1968), with the following reaction stoichiometry :where GSSG is oxidised glutathione.This enzyme plays a major role in the prevention of hydroperoxide-induced alterations of cellular structures (Flohe et al., 1973).SeGPx is a tetrameric protein containing four identical subunits, each with a molecular mass of approximately 20 kDa. Its GPx activity depends on the presence of selenium at the active site (Rotruck et al., 1973; Flohe et al., 1973). Selenium was shown to be incorporated in the protein backbone in the form of selenocysteine (SeCys). A ping-pong mechanism. in which the active-site selenium of the enzyme alternates between oxidized and reduced forms, was demonstrated (FlohC, 1971), but the intermediates involved in this catalytic cycle have not been fully elucidated.Correspondence to C. Rocher, Centre de Recherches Roussel-UCLAF, 107 -1 11 route de Noisy, F-93230 Romainville, France .4bbreiGotiom. CHPO, cumene hydroperoxide; FDH, formate dehydrogenase; GSSG, glutathione disulfide; GPx, glutathione peroxidase; GSH, reduced glutathione; /ucIq, luclpromoter mutation which confers a high level of lac repressor; SeCys, selenocysteine; SeGPx. selenium-glutathione peroxidase; tBHPO, t-butyl hydroperoxidc; ROOH. organic hydroperoxide; IAA, iodoacetate.Enqwie. Selenium-glutathione peroxidase.

Interleukin‐1 beta converting enzyme (ICE) is composed of 10′ (p10) and 20 kDa (p20) subunits, which are derived from a common 45 kDa precursor. Recent crystallographic studies have shown that ICE exists as a tetramer (p20/p10)2 in the crystal lattice. We provide evidence that the p10 and p20 subunits of ICE associate as oligomers in transfected COS cells. Using intragenic complementation, we show that the activity of a p10/p10 interface mutant defective in autoprocessing can be restored by co‐expression with active site ICE mutants. Different active site mutants can also complement each other by oligomerization to form active ICE. These studies indicate that ICE precursor polypeptides may associate in different quaternary structures and that oligomerization is required for autoprocessing. Furthermore, integenic complementation of active site mutants of ICE and an ICE homolog restores autoprocessing activity, suggesting that hetero‐oligomerization occurs between ICE homologs.

The complete nucleotide sequences of three cDNAs coding for the C-terminal part of mouse histocompatibility (H-2) antigens, and for the 3' non coding regions of these clones have been determined. Comparison of the sequences indicates a large homology throughout the coding and non-coding regions and suggests the existence of a genetic mechanism which homogenizes nucleotide sequences among genes of the H-2 multigene family.

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