Role of progesterone on peri-implantation stage endometrium-embryo interaction in the primate

Progesterone secretion during the luteal phase influences oviductal and endometrial functions which are essential for embryo viability and implantation in a number of species including primates. Luteal phase estrogen is not essential for progesterone-dependent endometrial receptivity towards implantation and pregnancy in the rhesus monkey and in the human. However, synchronous development of embryo and endometrium is an essential prerequisite for evolutive implantation. Progesterone helps to maintain synchronous development of preimplantation embryo through its action on maternal uterus. The anti-nidatory action of mifepristone, a potent progesterone receptor modulator (PRM) with pronounced antiprogestagenic activity, is known to be associated with desynchronization of endometrium along with repression of glandular secretory differentiation and vascular maturation. Thus, it is likely that early luteal phase administration of mifepristone affects paracrine action of the secretory stage endometrium on the preimplantation stage embryo, and thereby inhibits embryonic development and viability. We shall examine this hypothesis using the rhesus monkey as a primate model.

Expression of vascular endothelial growth factor (VEGF) and placental growth factor (PlGF) in conceptus and endometrium during implantation in the rhesus monkey

The aim of this study was to analyse the expression of transcripts and proteins for vascular endothelial growth factor (VEGF) and placenta growth factor (PlGF) in different compartments of the early conceptus at primary implantation sites during lacunar (n = 6), early villous (n = 9) and villous placenta (n = 6) stages of gestation in the rhesus monkey. During the lacunar stage, VEGF expression was observed in the cytotrophoblast cells lining the extraembryonic cavity, but these cells did not express PlGF. With further development, cytotrophoblast cells lining villi, forming columns, and constituting anchoring villi, expressed both VEGF and PlGF during early villous and villous placenta stages. In addition, chorion, amnion and villous stromal cells expressed both VEGF and PlGF proteins and mRNA. During the lacunar stage, all epithelial cells in maternal endometrium generally expressed VEGF, while PlGF expression was observed in the plaque epithelium only. As gestation advanced, the expression of VEGF and PlGF from plaque cells decreased, and in surface and glandular epithelium the expression of VEGF increased, while the expression of PlGF remained unaltered. Decidual stromal cells expressed VEGF and PlGF only at low levels during the lacunar stage, while the expression of both increased during the early villous and the villous placenta stages of implantation. It appears from the present study that the expression of VEGF and PlGF are regulated in a temporal and spatial manner during early stages of implantation and that their concerted actions in placental and maternal compartments play a critical role in the evolving pregnancy in the rhesus monkey.

High throughput and global approaches to gene expression

In the past several years, a new set of technologies based on whole genome analysis have revolutionized the study of gene expression. These microarray or "gene chip" technologies, which arose out of the development of large-scale sequencing approaches, are now coming into increasing use, generating a far greater volume of data than the data representing the sequences themselves. This review focuses on the current state of development of these technologies, and the available approaches to manage and analyze the information they generate. The applicability of this technology to general problems in biomedicine is also discussed.

Anti-nidatory effect of vaginally administered fumagillin in the rhesus monkey

In the present study, fumagillin, which is an antibiotic with potent angiostatic activity secreted from Aspergillus fumigatus was administered intravaginally during peri-implantation stage in the rhesus monkey and its effects on ovarian function, blastocyst implantation and pregnancy outcome in the rhesus monkey were investigated. Female monkeys (n = 18) showing normal menstrual cycles were vaginally inserted with tampons containing fumagillin (0 mg/animal in group 1; 1 mg/animal in group 2; 2 mg/animal in group 3; 4 mg/animal in group 4) on cycle day 20 of the mated treatment cycle, and these were removed on day 26 of the treatment cycle. Pregnancy was found to occur in animals treated with 1 mg and 2 mg fumagillin. However, animals treated with 4 mg fumagillin remained non-pregnant along with decreased (p <0.001) concentration of progesterone in circulation during the luteal period compared with that in normal, non-mated, ovulatory cycle.

Anti-nidatory effect of vaginally administered (Ala8,13, 18)-magainin II amide in the rhesus monkey

The hypothesis that timed application of a potent anti-microbial, anti-tumor agent like magainin peptides can inhibit blastocyst implantation was examined in the present study using the rhesus monkey as the primate model. Incidence of pregnancy, vaginal bleeding patterns, serum levels of progesterone, estrogen and monkey chorionic gonadotropin were examined following vaginal administration of (Ala8,13,18)-magainin II amide, a synthetic analogue of magainin 2, via tampon during days 20 to 26 of mated cycles. Implantation occurred in two out of three animals following administration of 0.25 mg magainin, while administration of 0.5 mg (Ala8,13,18)-magainin II amide resulted in inhibition of implantation in all females with no change in lengths of treatment cycles, and subsequent cycles. It appears from the present study that, besides being a local microbicidal agent, intravaginal administration of (Ala(8,13,18))-magainin II amide is a potential anti-implantation strategy for intercepting pregnancy.

Estrogen metabolism as a regulator of estrogen action in the mammary gland

Estrogen action in the target cells is dependent on estrogen receptor activity and intracellular estrogen concentration, which, in turn, is affected by the serum concentration and local metabolism in these cells. During the reproductive years the main source of estrogens is the ovarian follicles, but in postmenopausal women most of the estrogens are formed in peripheral tissues. 17Beta-hydroxysteroid dehydrogenases (17HSDs) catalyze the reaction between 17beta-hydroxysteroids and 17-ketosteroids, and several distinct 17HSD isoenzymes have been characterized. 17HSD type 1 catalyzes the reaction from low-activity estrone to high-activity estradiol. The type 2 enzyme has an opposite activity, thereby reducing the exposure of tissues to estrogen action. 17HSD type 1 is expressed both in steroidogenic tissues and in the target tissues of steroid action, such as normal and malignant breast tissue, where it may be responsible for maintaining the high intracellular estradiol concentration seen in breast cancer specimens. Therefore, 17HSD type 1 inhibitors may be useful in the treatment and/or prevention of estrogen-dependent malignancies, such as breast cancer. This article deals mainly with 17HSD types 1 and 2 and their role in estrogen action in breast tissue.

Nonparametric analysis of recurrent events and death

This article is concerned with the analysis of recurrent events in the presence of a terminal event such as death. We consider the mean frequency function, defined as the marginal mean of the cumulative number of recurrent events over time. A simple nonparametric estimator for this quantity is presented. It is shown that the estimator, properly normalized, converges weakly to a zero-mean Gaussian process with an easily estimable covariance function. Nonparametric statistics for comparing two mean frequency functions and for combining data on recurrent events and death are also developed. The asymptotic null distributions of these statistics, together with consistent variance estimators, are derived. The small-sample properties of the proposed estimators and test statistics are examined through simulation studies. An application to a cancer clinical trial is provided.

Methods for analysis of multiple events in the presence of death

The use of multiple events data in the design and analysis of clinical trials is becoming quite common. Several methods for analyzing such data have been proposed in the literature. However, they do not account for deaths in the analysis. In this article, an overview of available methods for modeling recurrent events and death is given. These methods are illustrated using data from an HIV clinical trial.

Gelastic epilepsy possibly following antirabies vaccine

A case of child with gelastic epilepsy following administration of antirabies vaccine is presented. No associated structural brain could be identified in the patient. Possibility of relationship between antirables vaccine and gelastic epilepsy is discussed.

Steroid dehydrogenase structures, mechanism of action, and disease

Steroid dehydrogenase enzymes influence mammalian reproduction, hypertension, neoplasia, and digestion. The three-dimensional structures of steroid dehydrogenase enzymes reveal the position of the catalytic triad, a possible mechanism of keto-hydroxyl interconversion, a molecular mechanism of inhibition, and the basis for selectivity. Glycyrrhizic acid, the active ingredient in licorice, and its metabolite carbenoxolone are potent inhibitors of human 11 beta-hydroxysteroid dehydrogenase and bacterial 3 alpha, 20 beta-hydroxysteroid dehydrogenase (3 alpha, 20 beta-HSD). The three-dimensional structure of the 3 alpha, 20 beta-HSD carbenoxolone complex unequivocally verifies the postulated active site of the enzyme, shows that inhibition is a result of direct competition with the substrate for binding, and provides a plausible model for the mechanism of inhibition of 11 beta-hydroxysteroid dehydrogenase by carbenoxolone. The structure of the ternary complex of human 17 beta-hydroxysteroid dehydrogenase type 1 (17 beta-HSD) with the cofactor NADP+ and the antiestrogen equilin reveals the details of binding of an inhibitor in the active site of the enzyme and the possible roles of various amino acids in the catalytic cleft. The short-chain dehydrogenase reductase (SDR) family includes these steroid dehydrogenase enzymes and more than 60 other proteins from human, mammalian, insect, and bacterial sources. Most members of the family contain the tyrosine and lysine of the catalytic triad in a YxxxK sequence. X-ray crystal structures of 13 members of the family have been completed. When the alpha-carbon backbone of the cofactor binding domains of the structures are superimposed, the conserved residues are at the core of the structure and in the cofactor binding domain, but not in the substrate binding pocket.

Macrophage protein kinase C: its role in modulating membrane microviscosity and superoxide in leishmanial infection

Pretreatment of macrophages with, an agonist of PKC, showed diverse effects on degradation and survival of two virulent strains of Leishmania donovani promastigotes. Treatment of macrophages with PMA for 45 min at 37 degrees C generated significant amounts of superoxide anions and reduced the parasite burden of macrophages by up to 48 and 43% when AG83 and GE-1 strains were used for infection. Staurosporine, an inhibitor of PKC, inhibited PMA-dependent killing of the parasites, while tyrphostin AG 126, an inhibitor of protein tyrosine kinase, showed very little effect. Depletion of PKC by prolonged incubation with PMA drastically reduced the superoxide anion generation and increased the uptake and multiplication of the parasites. Finally, to understand the mechanism of higher uptake of the parasites by PKC-depleted macrophages, membrane microviscosity was measured by fluorescence depolarization. Membrane microviscosity was found to be approximately 40% lower in PKC-depleted macrophages than in normal macrophages, indicating the role of membrane fluidity in the infection process. Together, these data suggest PKC activation, superoxide generation, and membrane fluidity are essential factors in the efficient regulation of leishmanial infection.

Preimplantation embryo morphology following early luteal phase anti-nidatory treatment with mifepristone (RU486) in the rhesus monkey

The ultrastructural characteristics of peri-implantation stage embryos recovered on day 6 after ovulation from rhesus monkeys with or without mifepristone (RU486) treatment during the early luteal phase were examined in the present study. Monkeys were randomly allocated to two groups; group 1 animals were injected s.c. with 2 ml vehicle (1:4, benzyl benzoate: olive oil, v/v, n = 21) and group 2 animals received a single dose of mifepristone (2 mg/kg body weight, w/v, n = 30) in the same volume of vehicle on day 2 after ovulation in mated cycles. On day 6 after ovulation, female monkeys of both groups were laparotomized and their reproductive tracts were flushed to retrieve preimplantation stage embryos. Embryos that showed frank degeneration or desynchrony on gross microscopical examination were not included in the present study. Preimplantation embryo growth on day 6 after ovulation was significantly (P < 0.05) affected in the morula-blastocyst transition stage in mifepristone-treated monkeys compared with that in the control group of monkeys. Ultrastructurally, administration of mifepristone on day 2 after ovulation depressed preimplantation stage embryo development, characterized by loss of cell polarity, lack of mitochondrial maturity, and lack of differentiation in trophoblast cells. Furthermore, preimplantation embryos from mifepristone-treated animals displayed a higher occurrence of inter-blastomere space, intra-cytoplasmic vacuoles, myelinoid bodies, accumulation of lipid droplets, lysosomes, lipofuscins, autophagosomes and multivesicular bodies. Collectively, it appears that the developmental potential of preimplantation embryos was significantly compromised in mifepristone-treated cycles.

Object-oriented transcription factors database (ooTFD)

ooTFD (object-oriented Transcription Factors Database) is an object-oriented successor to TFD. This database is aimed at capturing information regarding the polypeptide interactions which comprise and define the properties of transcription factors. ooTFD contains information about transcription factor binding sites, as well as composite relationships within transcription factors, which frequently occur as multisubunit proteins that form a complex interface to cellular processes outside the transcription machinery through protein-protein interactions. In the past year, a few additions and changes were made to this database and associated tools, which are accessible through the IFTI-MIRAGE web site at http://www.ifti.org/

Effect of sodium arsenite on plasma levels of gonadotrophins and ovarian steroidogenesis in mature albino rats: duration-dependent response

Effect of arsenic on ovarian steroidogenesis at the dose available in drinking water at wide areas of West Bengal is reported here. Weights of ovary, uterus and vagina along with biochemical activities of ovarian delta 5-3 beta-hydroxysteroid dehydrogenase (delta 5-3 beta-HSD) and 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) and plasma levels of LH, FSH and estrogen were measured in mature rats of the Wistar strain at diestrous phase following subchronic treatment with sodium arsenite at a dose of 0.4 ppm/rat/day for 16 days (4 estrous cycles) and 28 days (7 estrous cycles). A significant reduction in plasma levels of LH, FSH and estrogen along with significant diminution in the activities of ovarian delta 5-3 beta-HSD and 17 beta-HSD were observed following sodium arsenite treatment for 28 days. This duration of treatment also resulted in a marked degree in diminution in the weights of ovary, uterus and vagina, but 16 days of treatment did not exhibit any significant effect on these above parameters. Arsenic-treated rats exhibited a prolonged diestrous phase in the estrous cycle in contrast to control rats having 4 days of a regular estrous cycle. Deposition of arsenic in ovary, uterus, vagina and plasma was also monitored in arsenic-treated rats. The results of our experiment suggest that duration of arsenic treatment is the critical factor for its adverse effect on ovarian activities at the dose within the range noted in drinking water at several areas of West Bengal in India.

Statistical methods for analyzing repeated measures

Repeated measurements arise frequently in biomedical research. In many situations, the scientific question of interest concerns finding differences in the measurements between groups. This question is frequently addressed by using analysis of variance-type methods that fail to incorporate information regarding the repeated sampling design of the experiment. In this paper, we provide an introduction to nonstatisticians of two approaches for analyzing such data. The procedures can be performed by using available software. These methods are illustrated on data from a preclinical study conducted by ZymoGenetics.

Investigation of an epidemic of Reye's syndrome in northern region of India

OBJECTIVE

To determine the extent, epidemiological and clinical features of an epidemic of non-inflammatory encephalopathy in northern region of India.

DESIGN

Surveillance of referred cases having unconsciousness after a short bout of fever during October and November 1997. Case control study in 7 most affected villages.

METHODS

Active case finding was done to assess the extent and severity of the epidemic by interviewing health professionals and by reviewing mortality records in 10 districts of Haryana, Punjab and Chandigarh. A house to house survey was conducted in seven most affected villages. A case was defined as any child of less than 15 years of age, who had prodromal fever followed by vomiting and unconsciousness with subsequent recovery or death. Two age and sex matched controls who had fever without unconsciousness were taken for each case, one from nearby house and another staying furthest from the affected house. These groups were compared for various epidemiologic factors, clinical features and treatment pattern. Residual medicines used by affected patients were tested for presence of salicylate. Local village practitioners were interviewed for their knowledge and attitude towards use of aspirin in a febrile child.

RESULTS

Information regarding 129 affected children (M: F=1 : 1) could be obtained. Age ranged between 1 to 12 years (mean 5.8 years). Most were from rural or semi-suburban areas. Attack rate was 5.4/1000 and case fatality rate was 72%. Multiple sibs were affected in 9.3%. History of fever was reported by 83%, vomiting preceding unconsciousness by 83% and abnormal behavior by 65%. Abnormal posturing was reported in 55%. Seventeen (61%) of 28 samples had IgM antibodies in serum/CSF against measles. Twelve (36%) of 33 serum samples tested positive for Varicella zoster virus. None gave history of aspirin intake and 10 samples of residual drugs did not contain salicylate. However, 6 out of 19 blood samples taken from affected patients contained salicylate. Environmental factors were in favor of Japanese encephalitis (JE) but brain biopsy and serology disproved it. Based on earlier report of JE from this area, the cases in present epidemic were being reported as JE before this study was undertaken. Intensive fogging with malathion was being undertaken as antimosquito measure, specially around the affected houses. Local village practitioners (n = 37) were unaware of contraindications of aspirin in a febrile child.

CONCLUSION

Measles and varicella zoster emerged as the probable etiologies for the viral prodrome precipitating these cases of Reye's syndrome. Aspirin might have a contributory role. Malathion is another putative cofactor.

Co-occupancy of the interferon regulatory element of the class II transactivator (CIITA) type IV promoter by interferon regulatory factors 1 and 2

Class II transactivator (CIITA) activates the expression of major histocompatibility class II genes, which encode antigen-presenting molecules recognized by the T-cell receptor of CD4+ T cells. IFN-gamma induced CIITA transcription in many cell types is directed by the CIITA Type IV promoter. Here we report that the human CIITA Type IV promoter IRF-E binds IRF-1 and can be activated by exogenous expression of IRF-1. Surprisingly, the CIITA Type IV promoter IRF-E is also activated by IRF-2, another member of the IRF family that generally acts as a transcriptional repressor. In addition, we found that IRF-1 and IRF-2 synergistically activate the CIITA Type IV promoter. Electrophoretic mobility shift assays revealed that IRF-1 and IRF-2 can simultaneously occupy the IRF-E of the CIITA Type IV promoter, suggesting a novel mechanism for the role of these two proteins in promoter activation. Our results also indicate that IRF-1 and IRF-2 can cooperatively activate and co-occupy the IRF-E of the guanylate binding protein (GBP) promoter. Finally, CIITA induction by IFN-gamma does not occur in a pancreatic tumor cell line that expresses a mutated IRF-2, representing the first IRF-2 mutation identified in a human tumor cell line.

Methoxychlor stimulates estrogen-responsive messenger ribonucleic acids in mouse uterus through a non-estrogen receptor (non-ER) alpha and non-ER beta mechanism

This study examined the effects of the xenoestrogen methoxychlor (Mxc) on messenger RNA (mRNA) concentrations of two estrogen-responsive uterine genes, lactoferrin (LF) and glucose-6-phosphate dehydrogenase (G6PD). Ovariectomized wild-type (WT) and estrogen receptor (ER) alpha-knockout (ER alphaKO) mice were treated with Mxc or estradiol-17beta (E2) to determine whether Mxc acts via pathways that involve ER alpha. In WT mice, both E2 and Mxc stimulated increases in uterine LF and G6PD mRNA concentrations in a dose-dependent manner. Competitive pretreatment with the pure antiestrogen ICI 182,780 dramatically reduced E2-stimulated increases in mRNA concentrations but had no effect on Mxc-induced effects. Competitive pretreatment with E2 had only a partially inhibitory effect on Mxc-induced responses. In the ER alphaKO mouse, E2 had little effect on uterine LF or G6PD mRNA concentrations, whereas Mxc stimulated marked increases in both LF and G6PD mRNAs. The Mxc-induced increases in LF and G6PD mRNAs in the ER alphaKO mouse were not suppressed by competitive pretreatment with either E2 or ICI 182,780. Fold increases in mRNA concentrations for both genes induced by Mxc were similar for WT and ER alphaKO mice. The results surprisingly indicate that a xenoestrogen, Mxc, can increase LF and G6PD mRNA concentrations by a mechanism that is not mediated through ER alpha or ER beta, and acts through another pathway.

Effect of ascorbic acid supplementation on liver and kidney toxicity in cyclophosphamide-treated female albino rats

Effects of ascorbic acid supplementation on the activity of acid phosphatase (ACP), alkaline phosphatase (ALP), glutamic oxaloacetic transaminase (GOT) and glutamic pyruvic transaminase (GPT) on liver, kidney and serum in cyclophosphamide-treated female virgin rats were investigated. Oral administration of cyclophosphamide at the dose of 5 mg/kg body weight/day for 12 days resulted in a significant elevation in ACP and ALP activities in liver, kidney and serum. Ascorbic acid supplementation at the dose of 25 mg/kg body weight/day showed a significant protection in the activity of ACP in liver, kidney and serum, but only in ALP activity in kidney. ALP activities in liver and serum were not restored to control level by ascorbic acid supplementation. Activities of GOT and GPT were elevated significantly in liver, kidney and serum after cyclophosphamide treatment, and were protected and restored to control level by ascorbic acid supplementation.

Structure of Ustilago maydis killer toxin KP6 alpha-subunit. A multimeric assembly with a central pore

Ustilago maydis is a fungal pathogen of maize, some strains of which secrete killer toxins. The toxins are encoded by double-stranded RNA viruses in the cell cytoplasm. The U. maydis killer toxin KP6 contains two polypeptide chains, alpha and beta, having 79 and 81 amino acids, respectively, both of which are necessary for its killer activity. The crystal structure of the alpha-subunit of KP6 (KP6alpha) has been determined at 1.80-A resolution. KP6alpha forms a single domain structure that has an overall shape of an ellipsoid with dimensions 40 A x 26 A x 21 A and belongs to the alpha/beta-sandwich family. The tertiary structure consists of a four-stranded antiparallel beta-sheet, a pair of antiparallel alpha-helices, a short strand along one edge of the sheet, and a short N-terminal helix. Although the fold is reminiscent of toxins of similar size, the topology of KP6alpha is distinctly different in that the alpha/beta-sandwich motif has two right-handed betaalphabeta split crossovers. Monomers of KP6alpha assemble through crystallographic symmetries, forming a hexamer with a central pore lined by hydrophobic N-terminal helices. The central pore could play an important role in the mechanism of the killing action of the toxin.

Characterization of molecular and catalytic properties of intact and truncated human 17beta-hydroxysteroid dehydrogenase type 2 enzymes: intracellular localization of the wild-type enzyme in the endoplasmic reticulum

Human 17beta-hydroxysteroid dehydrogenase (17HSD) type 2 is a widely distributed enzyme that primarily converts the highly active 17beta-hydroxysteroids to their inactive keto forms. In the present study, full-length human 17HSD type 2 was localized in the endoplasmic reticulum using a double immunofluorescence labeling technique. As a consequence of its strong membrane interaction, full-length human 17HSD type 2 could not be solubilized as a biologically active form in vitro. However, by deleting the first 29 amino acids from the N-terminus, we were able to purify a catalytically active enzyme from the cytosolic fraction of Sf9 insect cells. Biochemical and catalytic properties of the purified truncated human 17HSD type 2 protein confirm its suitability for structure-function analyses of the enzyme. Both intact and truncated 17HSD type 2 enzymes efficiently catalyzed the oxidation of estradiol, testosterone, dihydrotestosterone, androstenediol, and 20alpha-dihydroprogesterone. The oxidation of estradiol brought about by human 17HSD type 2 was effectively inhibited by several other steroidal compounds, such as 2-hydroxyestradiol, 5beta-androstan-3alpha,17beta-diol, 5alpha-androstan-3alpha,17beta-diol, and 5alpha-androstan-3beta,17beta-diol. The broad substrate specificity of human 17HSD type 2 together with its predominant oxidative activity and intracellular location, as observed in this study, indicate the physiological role of the enzyme to be primarily an inactivator of highly active 17beta-hydroxysteroids.

Structure of an activity suppressing Fab fragment to cytochrome P450 aromatase: insights into the antibody-antigen interactions

The crystal structure of a Fab fragment (Fab3-2C2) of a monoclonal antibody raised against aromatase cytochrome P450 P450arom) has been determined at 3.0 A resolution. P450arom is a membrane bound enzyme responsible for the catalysis of indrogens to estrogens, the process of aromatization, and hence has been implicated in hormone-dependent breast cancer. The Fab fragment of MAb3-2C2 IgG suppresses P450arom activity in a dose dependent manner. The Fab3-2C2 molecule crystallizes n the space group P2(1)2(1)2(1) with a unit cell of a= 154.89 A, b = 73.51 A, and c= 36.90 A. The crystal structure consists of a light and a heavy chain in the asymmetric unit, each characterized by the greek-key antiparallel beta barrel folding seen in all Fab structures. The average elbow angle between the two domains is 143 degrees. Modeling of the interactions between the variable domains of the antibody and a known model of P450arom maps the epitope to a region of the enzyme that is consistent with the available biochemical data and the activity-suppressing function of the antibody. The epitope mapping result is further supported by the inability of MAb3-2C2 IgG to suppress the activity of, or to interact with placental porcine P450arom, which is 81% identical (86% similar) to human P450arom but has a few key substitutions in the putative epitope region.

Determination of a protein structure by iodination: the structure of iodinated acetylxylan esterase

Enzymatic and non-enzymatic iodination of the amino acid tyrosine is a well known phenomenon. The iodination technique has been widely used for labeling proteins. Using high-resolution X-ray crystallographic techniques, the chemical and three-dimensional structures of iodotyrosines formed by non-enzymatic incorporation of I atoms into tyrosine residues of a crystalline protein are described. Acetylxylan esterase (AXE II; 207 amino-acid residues) from Penicillium purpurogenum has substrate specificities towards acetate esters of D-xylopyranose residues in xylan and belongs to a new class of alpha/beta hydrolases. The crystals of the enzyme are highly ordered, tightly packed and diffract to better than sub-angström resolution at 85 K. The iodination technique has been utilized to prepare an isomorphous derivative of the AXE II crystal. The structure of the enzyme determined at 1.10 A resolution exclusively by normal and anomalous scattering from I atoms, along with the structure of the iodinated complex at 1.80 A resolution, demonstrate the formation of covalent bonds between I atoms and C atoms at ortho positions to the hydroxyl groups of two tyrosyl moieties, yielding iodotyrosines.