Methane monooxygenase (MMO), found in aerobic methanotrophic bacteria, catalyzes the 02-dependent conversion of methane to methanol. The soluble form of the enzyme (sMMO) consists of three components: a reductase, a regulatory "B" component (MMOB), and a hydroxylase component (MMOH), which contains a hydroxo-bridged dinuclear iron cluster. Two genera of methanotrophs, termed Type X and Type 11, which differ markedly in cellular and metabolic characteristics, are known to produce the sMMO. The structure of MMOH from the Type X methanotroph Methylococcus capsulatus Bath (MMO Bath) has been reported recently. Two different structures were found for the essential diiron cluster, depending upon the temperature at which the diffraction data were collected. In order to extend the structural studies to the Type I1 methanotrophs and to determine whether one of the two known MMOH structures is generally applicable to the MMOH family, we have determined the crystal structure of the MMOH from Type I1 Methylosinus trichosporium OB3b (MMO OB3b) in two crystal forms to 2.0 A and 2.7 8, resolution, respectively, both determined at 18 "C. The crystal forms differ in that MMOB was present during crystallization of the second form. Both crystal forms, however, yielded very similar results for the structure of the MMOH. Most of the major structural features of the MMOH Bath were also maintained with high fidelity. The two irons of the active site cluster of MMOH OB3b are bridged by two OH (or one OH and one H20), as well as both carboxylate oxygens of Glu a144. This bis-yhydroxo-bridged "diamond core" structure, with a short Fe-Fe distance of 2.99 A, is unique for the resting state of proteins containing analogous diiron clusters, and is very similar to the structure reported for the cluster from flash frozen (-160 "C) crystals of MMOH Bath, suggesting a common active site structure for the soluble MMOHs. The high-resolution structure of MMOH OB3b indicates 26 consecutive amino acid sequence differences in the p chain when compared to the previously reported sequence inferred from the cloned gene. Fifteen additional sequence differences distributed randomly over the three chains were also observed, including Da209E, a ligand of one of the irons.

Contamination of arsenic in drinking water is associated with several human diseases including cancer. It has been reported that oxidative stress plays a vital role in arsenic-induced biochemical and molecular alterations. The aim of the present study was to improve the understanding of arsenic-induced oxidative damage to proteins and to DNA and the role of antioxidants such as ascorbic acid and α-tocopherol in alleviating arsenic-induced damages in experimental rats. A significant increase in the levels of protein oxidation, DNA strand breaks, and DNA–protein cross-links was observed in blood, liver, and kidney of rats exposed to arsenic (100 ppm in drinking water) for 30 days. Co-administration of ascorbic acid and α-tocopherol to arsenic-exposed rats showed a substantial reduction in the levels of arsenic-induced oxidative products of protein and DNA. The results of this study support that free radical–mediated toxic manifestations of arsenic and also suggest that ascorbic acid and α-tocopherol supplementation can improve the arsenic-induced molecular alterations.

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It is hypothesized that a reduction in the level of antioxidants with age leads to an impairment in the quenching of free radicals, which in turn increases the risk of succumbing to ageassociated disorders. Male albino rats of Wistar strain (both young and aged rats) were treated with lipoic acid for 7 and 14 days. Analyses were carried out in blood, liver, kidney, and brain for lipid peroxidation, reduced glutathione, ascorbic acid, and a-tocopherol. The levels of reduced glutathione, and vitamins C and E were found to be lowered in aged rats, whereas the level of lipid peroxidation was found to be high. After intraperitoneal administration of lipoate (a thiol antioxidant) to the aged rats, a time-dependent reduction in the level of lipid peroxidation and elevation in the levels of reduced glutathione, and vitamins C and E were observed. From our observations, we conclude that lipoic acid, a dithiol, normalizes lipid peroxidation and prevents the oxidation of reduced glutathione, possibly through recycling mechanisms, thereby maintaining normal metabolic function. Thus, lipoic acid supplementation could be beneficial in minimizing age-associated disorders where free radicals are the major cause. 265

Two Fortran programs are described for conducting various statistical tests for centrosymmetry. The first program, STATCW, is for crystals that satisfy the requirements of basic Wilson distributions while the second, STATCN, is for any crystal for which the probability density functions of the normalized structure-factor-magnitude variable y for hypothesis testing are available only in the form of a numerical table. As well as seven quantitative statistical tests, the moments of truncated distributions are also included in these programs as test criteria and a unique decision is arrived at using suitable decision rules. The two programs were tested in a number of cases and typical results obtained are reported.

502MEAN ERROR IN ATOMIC POSITIONS described in § 4. In such hypothetical cases, I FNI will be free from errors of observation. However, when one uses the observed overall value of an R index for finding (Izarl), one must remember that the experimental errors in I Fobsl would also contribute to the overall value of the R index. Owing to this one has to note the following two points in estimating (Izarl)

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