Catalytic mechanism and role of hydroxyl residues in the active site of theta class glutathione S-transferases. Investigation of Ser-9 and Tyr-113 in a glutathione S-transferase from the Australian sheep blowfly, Lucilia cuprina

Spectroscopic and kinetic studies have been performed on the Australian sheep blowfly Lucilia cuprina glutathione S-transferase (Lucilia GST; EC 2.5.1.18) to clarify its catalytic mechanism. Steady state kinetics of Lucilia GST are non-Michaelian, but the quite hyperbolic isothermic binding of GSH suggests that a steady state random sequential Bi Bi mechanism is consistent with the anomalous kinetics observed. The rate-limiting step of the reaction is a viscosity-dependent physical event, and stopped-flow experiments indicate that product release is rate-limiting. Spectroscopic and kinetic data demonstrate that Lucilia GST is able to lower the pKa of the bound GSH from 9.0 to about 6.5. Based on crystallographic suggestions, the role of two hydroxyl residues, Ser-9 and Tyr-113, has been investigated. Removal of the hydroxyl group of Ser-9 by site-directed mutagenesis raises the pKa of bound GSH to about 7.6, and a very low turnover number (about 0.5% of that of wild type) is observed. This inactivation may be explained by a strong contribution of the Ser-9 hydroxyl group to the productive binding of GSH and by an involvement in the stabilization of the ionized GSH. This serine residue is highly conserved in the Theta class GSTs, so the present findings may be applicable to all of the family members. Tyr-113 appears not to be essential for the GSH activation. Stopped-flow data indicate that removal of the hydroxyl group of Tyr-113 does not change the rate-limiting step of reaction but causes an increase of the rate constants of both the formation and release of the GSH conjugate. Tyr-113 resides on alpha-helix 4, and its hydroxyl group hydrogen bonds directly to the hydroxyl of Tyr-105. This would reduce the flexibility of a protein region that contributes to the electrophilic substrate binding site; segmental motion of alpha-helix 4 possibly modulates different aspects of the catalytic mechanism of the Lucilia GST.

Homology model for the human GSTT2 Theta class glutathione transferase

A tertiary model of the human GSTT2 Theta class glutathione transferase is presented based on the recently solved crystal structure of a related thetalike isoenzyme from Lucilia cuprina. Although the N-terminal domains are quite homologous, the C-terminal domains share less than about 20% identity. The model is used to consolidate the role of Ser 11 in the active site of the enzyme as well as to identify other residues and mechanisms of likely catalytic importance. The T2 subfamily of theta class enzymes have been shown to inactivate reactive sulfate esters arising from arylmethanols. A possible reaction pathway involving the conjugation of glutathione with one such sulfate ester, 1-menaphthyl-sulfate, is described. It is also proposed that the C-terminal region of the enzyme plays an important role in allowing substrate access to the active site.

Identification of an essential cysteine residue in human glutathione synthase

Glutathione is essential for a variety of cellular functions, and is synthesized from gamma-glutamylcysteine and glycine by the action of glutathione synthase (EC 6.3.2.3). Human glutathione synthase is a dimer of two identical subunits, each composed of 474 amino acids. Little is known about the structure-function relationships of mammalian glutathione synthases and, in order to gain a greater understanding of this critical enzyme, we have probed the role of cysteine residues by chemical modification and site-directed mutagenesis. Preincubation with thiol reagents such as p-chloromercuribenzoate, N-ethylmaleimide, iodoacetate and 5,5'-dithiobis-(2-nitrobenzoate) resulted in significant inhibition of recombinant human glutathione synthase. Each subunit contains cysteine residues at positions 294, 409 and 422, and we have prepared four different mutants by replacing individual cysteine residues, or all of the cysteine residues, with alanine. The C294A and C409A cysteine mutants retained significant residual activity, indicating that these two cysteine residues are not essential for activity. In contrast, substantial decreases in enzymic activity were detected with the C422A and cysteine-free mutants. This suggests that Cys-422 may play a significant structural or functional role in human glutathione synthase.

Application of HUMF13A01 (AAAG)n STR polymorphism to the genetic diagnosis of coagulation factor XIII deficiency

Deficiency of the A subunit of coagulation factor XIII causes a severe bleeding disorder requiring life long replacement therapy. The mutations causing A subunit deficiency appear to be very heterogeneous, and it is impractical to identify each mutation before genetic counselling or prenatal diagnosis can be attempted. In this study we have shown that a highly polymorphic short tandem repeat element, HUMF13A01 (AAAG)n that occurs in the 5' flanking sequence of the factor XIII A subunit gene, can be used to follow the segregation of deficiency causing mutations. We studied 6 families with factor XIII A subunit deficiency from 5 different ethnic groups. All parents were heterozygous for the repetitive element and therefore all the families were informative. The linked polymorphism was used to carry out the first prenatal diagnosis of factor XIII A subunit deficiency. The analysis of this polymorphism by the polymerase chain reaction is rapid, reliable, requires little DNA and is ideal for the genetic analysis of factor XIII A subunit deficiency.

Mutagenesis of the active site of the human Theta-class glutathione transferase GSTT2-2: catalysis with different substrates involves different residues

The role of serine-11 in the catalytic mechanism of recombinant human GSTT2-2 was examined by site-directed mutagenesis. Amino acid sequence comparison of the Theta-class isoenzymes has identified a conserved serine residue in the N-terminal domain [Wilce, Board, Feil and Parker (1995) EMBO J. 14, 2133-2143]. This conserved serine has been implicated in the activation of the enzyme-bound glutathione [Board, Coggan and Parker (1995) Biochem. J. 311, 247-250]. Mutating the equivalent serine (residue 11) of GSTT2-2 to Ala, Thr or Tyr abolished the catalytic properties of GSTT2-2 with cumene hydroperoxide and ethacrynic acid as second substrate. However, with l-menaphthyl sulphate (MSu) as the second substrate, the specific activity of the S11A mutant was doubled, while the S11T mutant retained half the wild-type activity and the S11Y mutant was inactive. The role of Ser-11 in catalysis seems to vary with different second substrates. In the substitution reaction with MSu, GSTT2-2 activity appears to depend on the size of the Ser-11 replacement rather than the presence of a side-chain hydroxy group. In addition, the reaction rate appears to be a function of pH, and there is no non-enzymic reaction even at high pH. We demonstrated that a reaction between MSu and an alternative thiol such as L-cysteine or 2-mercaptoethanol can take place in the presence of S-methylglutathione and GSTT2-2. We propose that the catalytic activity of GSTT2-2 with MSu is preceded by a conformational or charge modification to the enzyme upon the binding of glutathione or S-methylglutathione. This is followed by the binding of MSu and the subsequent removal of the sulphate group, giving rise to the carbonium ion of l-methylnaphthelene as the electrophile that reacts with the nucleophilic species. The reaction mechanism of GSTT2-2 with MSu may represent a novel function of GSTT2-2 as a glutathione-dependent sulphatase.

New mutations causing the premature termination of translation in the A subunit gene of coagulation factor XIII

The amplification of factor XIII A subunit gene exons and heteroduplex analysis has been used to identify two new mutations that cause severe factor XIII deficiency. One mutation in a family of French origin results from a 4 bp deletion and leads to a premature termination of translation. The other mutation occurred in a Turkish family and results from a C-->T transition that inserts a premature translation stop signal at codon 400. Both mutations alter restriction enzyme sites and can be readily detected in amplified exon DNA for genetic counselling or prenatal diagnosis. The new mutations reflect the extensive molecular heterogeneity of factor XIII deficiency.

Purification and characterization of a recombinant human Theta-class glutathione transferase (GSTT2-2)

A cDNA encoding the human Theta-class glutathione transferase GSTT2-2 was expressed in Escherichia coli as a ubiquitin fusion protein. The co-translational removal of the ubiquitin by a cloned ubiquitin-specific protease, Ubp1, generates enzymically active GSTT2-2 without any additional N-terminal residues. The recombinant isoenzyme was purified to apparent homogeneity by DEAE anion-exchange, gel filtration, dye ligand chromatography and high resolution anion-exchange chromatography on Mono Q FPLC. The recombinant enzyme had significant activity with a range of substrates, including cumene hydroperoxide and 1-menapthyl sulphate. The activity of GSTT2-2 with a range of secondary lipid peroxidation products such as the trans,trans-alka-2,4-dienals and trans-alk-2-enals, as well as its glutathione peroxidase activity with organic hydroperoxides, suggest that it may play a significant role in protection against the products of lipid peroxidation.

A structurally derived consensus pattern for theta class glutathione transferases

We have recently determined the first crystal structure of a theta class glutathione transferase. We have aligned the amino acid sequences of members of the family using the crystal structure as a guide. The alignment has revealed a consensus pattern of residues that first identifies a protein as belonging to the glutathione transferase superfamily, and second is able to distinguish theta class members from other classes of glutathione transferases. The consensus residues unique to the theta class are found to cluster mostly on the hydrophilic surface and flanking loops of helix 2, a region found to be structurally diverse amongst crystal structures of the different glutathione transferase classes. When the consensus pattern was scanned against sequence databases, a number of matches were made with proteins not formally identified as glutathione transferases. Some of these matches indicated that several stress-related proteins belong to the theta class GST family.

Characterization of a cDNA and gene encoding the mouse theta class glutathione transferase mGSTT2 and its localization to chromosome 10B5-C1

In this study, we have isolated and characterized a gene and cDNA encoding a mouse Theta class GST. The gene, mGSTT2, spans approximately 3.1 kb and is composed of five exons interrupted by four introns. The gene was localized to Chromosome 10B5-C1 by in situ hybridization. Southern blot analysis of mouse genomic DNA suggests that there is only one copy of mGSTT2 in the mouse genome. The cDNA derived from mGSTT2 was isolated from a mouse liver cDNA library and has an open reading frame of 732 bp encoding a peptide of 244 amino acids with a calculated molecular weight of 26,676 Da. The encoded protein shares amino acid sequence identities of 92, 77, 51, and 55% with rat subunit Yrs, human subunit GSTT2, rat subunit 5, and human subunit GSTT1, respectively.

The gene encoding human glutathione synthetase (GSS) maps to the long arm of chromosome 20 at band 11.2

Two forms of glutathione synthetase deficiency have been described. While one form is mild, causing hemolytic anemia, the other more severe form causes 5-oxo-prolinuria with secondary neurological involvement. Despite the existence of two deficiency phenotypes, Southern blots hybridized with a glutathione synthetase cDNA suggest that there is a single glutathione synthetase gene in the human genome. Analysis of somatic cell hybrids showed the human glutathione synthetase gene (GSS) to be located on chromosome 20, and this assignment has been refined to subband 20q11.2 using in situ hybridization.

Evidence for an essential serine residue in the active site of the Theta class glutathione transferases

A consistent feature of the Alpha-, Mu- and Pi-class glutathione transferases (GSTs) is the presence near the N-terminus of a tyrosine residue that contributes to the activation of glutathione. While this residue appears to be conserved in many Theta-class GSTs, its absence in some suggested that the Theta-class GSTs may have a significantly different structure or catalytic mechanism. The elucidation of the crystal structure of the Theta-class GST from the Australian sheep blowfly, Lucilia cuprina, has indicated that a serine residue rather than a tyrosine residue can form a hydrogen bond with the glutathionyl sulphur atom. The present studies show that mutation of Ser-9 to alanine substantially inactivates the L. cuprina GST, confirming its importance in the reaction mechanism. As this serine is conserved in all Theta-class enzymes reported so far, it seems that an active-site serine is a significant factor that distinguishes the Theta-class GSTs from members of the Alpha-, Mu- and Pi-class isoenzymes.

Sequencing and expression of a cDNA for human glutathione synthetase

A human brain cDNA clone encoding glutathione synthetase (EC 6.3.2.3) has been sequenced and expressed in Escherichia coli. The protein is 474 amino acids in length with a subunit molecular mass of 52,352 Da. The recombinant protein exhibits glutathione synthetase activity and occurs as a homodimer. The recombinant glutathione synthetase was purified to homogeneity and had a specific activity of 1.73 mumol/min per mg of protein, an isoelectric point of 5.35 and a pH optimum between 7.0 and 7.5. Southern blots of human genomic DNA hybridized with the glutathione synthetase cDNA revealed a relatively simple pattern of strongly hybridizing fragments, indicating the absence of a large gene family and suggesting that there may be only one glutathione synthetase gene in the human genome.

Structural analysis of human alpha-class glutathione transferase A1-1 in the apo-form and in complexes with ethacrynic acid and its glutathione conjugate

BACKGROUND

Glutathione transferases (GSTs) constitute a family of isoenzymes that catalyze the conjugation of the tripeptide glutathione with a wide variety of hydrophobic compounds bearing an electrophilic functional group. Recently, a number of X-ray structures have been reported which have defined both the glutathione- and the substrate-binding sites in these enzymes. The structure of the glutathione-free enzyme from a mammalian source has not, however, been reported previously.

RESULTS

We have solved structures of a human alpha-class GST, isoenzyme A1-1, both in the unliganded form and in complexes with the inhibitor ethacrynic acid and its glutathione conjugate. These structures have been refined to resolutions of 2.5 A, 2.7 A and 2.0 A respectively. Both forms of the inhibitor are clearly present in the associated electron density.

CONCLUSIONS

The major differences among the three structures reported here involve the C-terminal alpha-helix, which is a characteristic of the alpha-class enzyme. This helix forms a lid over the active site when the hydrophobic substrate binding site (H-site) is occupied but it is otherwise disordered. Ethacrynic acid appears to bind in a non-productive mode in the absence of the coenzyme glutathione.

Crystal structure of a theta-class glutathione transferase

Glutathione S-transferases (GSTs) are a family of enzymes involved in the cellular detoxification of xenotoxins. Cytosolic GSTs have been grouped into four evolutionary classes for which there are representative crystal structures of three of them. Here we report the first crystal structure of a theta-class GST. So far, all available GST crystal structures suggest that a strictly conserved tyrosine near the N-terminus plays a critical role in the reaction mechanism and such a role has been convincingly demonstrated by site-directed mutagenesis. Surprisingly, the equivalent residue in the theta-class structure is not in the active site, but its role appears to have been replaced by either a nearby serine or by another tyrosine residue located in the C-terminal domain of the enzyme.

Functional significance of arginine 15 in the active site of human class alpha glutathione transferase A1-1

Arg15 is a conserved active-site residue in class Alpha glutathione transferases. X-ray diffraction studies of human glutathione transferase A1-1 have shown that N epsilon of this amino acid residue is adjacent to the sulfur atom of a glutathione derivative bound to the active site, suggesting the presence of a hydrogen bond. The phenolic hydroxyl group of Tyr9 also forms a hydrogen bond to the sulfur atom of glutathione, and removal of this hydroxyl group causes partial inactivation of the enzyme. The present study demonstrates by use of site-directed mutagenesis the functional significance of Arg15 for catalysis. Mutation of Arg15 into Leu reduced the catalytic activity by 25-fold, whereas substitution by Lys caused only a threefold decrease, indicating the significance of a positively charged residue at position 15. Mutation of Arg15 into Ala or His caused a substantial reduction of the specific activity (200 or 400-fold, respectively), one order of magnitude more pronounced than the effect of the Tyr9-->Phe mutation. Double mutations involving residues 9 and 15 demonstrated that the effects of mutations at the two positions were additive except for the substitution of His for Arg15, which appeared to cause secondary structural effects. The pKa value of the phenolic hydroxyl of Tyr9 was determined by UV absorption difference spectroscopy and was found to be 8.1 in the wild-type enzyme. The corresponding pKa values of mutants R15K, R15H and R15L were 8.5, 8.7 and 8.8, respectively, demonstrating the contribution of the guanidinium group of Arg15 to the electrostatic field in the active site. Addition of glutathione caused an increased pKa value of Tyr9; this effect was not obtained with S-methylglutathione. These results show that Tyr9 is protonated when glutathione is bound to the enzyme at physiological pH values. The involvement of an Arg residue in the binding and activation of glutathione is a feature that distinguishes class Alpha glutathione transferases from members in other glutathione transferase classes.

Molecular cloning of a cDNA and chromosomal localization of a human theta-class glutathione S-transferase gene (GSTT2) to chromosome 22

Until recently the Theta-class glutathione S-transferases (GSTs) were largely overlooked due to their low activity with the model substrate 1-chloro-2,4-dinitrobenzene (CDNB) and their failure to bind to immobilized glutathione affinity matrices. Little is known about the number of genes in this class. Recently, Pemble et al. (Biochem J. 300: 271-276, 1994) reported the cDNA cloning of a human Theta-class GST, termed GSTT1. In this study, we describe the molecular cloning of a cDNA encoding a second human Theta-class GST (GSTT2) from a lambda gt11 human liver 5'-stretch cDNA library. The encoded protein contains 244 amino acids and has 78.3% sequence identity with the rat subunit 12 and only 55.0% identity with human GSTT1. GSTT2 has been mapped to chromosome 22 by somatic cell hybrid analysis. The precise position of the gene was localized to subband 22q11.2 by in situ hybridization. The absence of other regions of hybridization suggests that there are no closely related sequences (e.g., reverse transcribed pseudogenes) scattered throughout the genome and that if there are closely related genes, they must be clustered near GSTT2. Southern blot analysis of human DNA digested with BamHI shows that the size of the GSTT2 gene is relatively small, as the coding sequence falls within a 3.6-kb BamHI fragment.

Site-directed mutagenesis of human glutathione transferase P1-1. Spectral, kinetic, and structural properties of Cys-47 and Lys-54 mutants

In the human placental glutathione transferase, Cys-47 possesses, at physiological pH values, a pK alpha value of 4.2 and may exist as an ion pair with the protonated epsilon-amino group of Lys-54. Using site-directed mutagenesis we investigate spectral, kinetic, and structural properties of Cys-47 and Lys-54 mutants. The results shown indicate that the thiolate ion detected at 229 nm should be assigned exclusively to Cys-47. The contribution of Lys-54 to the activation of Cys-47 is assessed by the spectral properties of the K54A mutant enzyme. The induced cooperativity toward glutathione, as a consequence of mutation of Lys-54 to alanine, clearly parallels that observed for the Cys-47 mutant enzymes (see the preceding paper (Ricci, G., Lo Bello, M., Caccuri, A. M., Pastore, A., Nuccetelli, M., Parker, M. W., and Federici, G. (1995) J. Biol. Chem. 270, 1243-1248) and points out the importance of this electrostatic interaction in shaping the correct spatial arrangement for the binding of glutathione and in anchoring the flexible helix alpha 2. When this ion pair is disrupted, by mutation of either residue, the flexibility of this region could be greatly increased, causing helix alpha 2 to come in contact with the other subunit and generating a structural communication, which is the basis of the observed cooperativity.

Protein expression using cotranslational fusion and cleavage of ubiquitin. Mutagenesis of the glutathione-binding site of human Pi class glutathione S-transferase

Expression of cloned genes in prokaryotes such as Escherichia coli is a widely used technique in both basic research and biotechnology. Despite the availability of several E. coli expression vector systems, adequate levels of expression may not be achieved. Expressing proteins as fusions to the highly conserved eukaryotic protein ubiquitin has been reported by several investigators to enhance protein yield in both bacterial and eukaryotic systems. We have modified this technique by the co-expression in E. coli of a ubiquitin-fusion protein and the Saccharomyces cerevisiae ubiquitin-specific protease Ubp2. This allows the co-translational cleavage of engineered ubiquitin-fusion proteins expressed in E. coli. This system was used to express a human Pi class glutathione S-transferase (GST) GSTP1 as well as two mutant GSTP1 derivatives, Trp39-->Cys and Gln52-->Glu. The yield of these enzymes was improved 40-fold by using the ubiquitin-fusion/co-translational cleavage technique, and no uncleaved product was detected. The Trp39-->Cys mutant was totally devoid of GST activity, while the activity of the Gln52-->Glu mutant was reduced to 6% of wild-type GSTP1-1. As both of the mutated residues map within the glutathione-binding site, the reduced GST activity is consistent with a marked reduction in glutathione binding ability.

Structure and function of the 5' flanking sequences of the human alpha class glutathione S-transferase genes

We have isolated the 5'flanking regions of two human Alpha class glutathione S-transferase genes, GSTA1 and GSTA2. The two genes share 95% sequence identity between nucleotide positions -1,300 and +500 from the transcriptional start site. Various DNA fragments from the 5' flanking region of the GSTA1 gene were fused to the chloramphenicol acetyltransferase reporter gene and transfected into HepG2 cells. The results indicated that negative regulatory and enhancer elements are located in the sequence upstream of the GSTA1 gene. Sequence analysis and functional assays have not found any evidence for xenobiotic- or antioxidant-responsive elements previously described in rodent Alpha class genes. Thus the transcriptional regulation of the human Alpha class glutathione S-transferase genes may be dramatically different from the regulation of Alpha class glutathione S-transferase genes in rodents.

Crystallization and preliminary X-ray diffraction studies of a glutathione S-transferase from the Australian sheep blowfly, Lucilia cuprina

Crystals of a glutathione S-transferase from the Australian sheep blowfly Lucilia cuprina have been grown from ammonium sulphate by the hanging drop vapour diffusion method. Successful crystallization required the presence of the inhibitor S-hexylglutathione. The crystals belong to the tetragonal space group P4(1)22 (or P4(3)22) with cell dimensions of a = b = 88.1 A and c = 66.9 A. They contain one monomer in the asymmetric unit and diffract beyond 2.8 A resolution.

Localization of the human UBA52 ubiquitin fusion gene to chromosome band 19p13.1-p12

Because of the conservation of the ubiquitin coding sequence and the number of transcriptionally active genes and reverse-transcribed pseudogenes, it has not been possible to use ubiquitin cDNA clones to map the functional ubiquitin genes. The UBB and UBC polyubiquitin genes have previously been mapped by the use of specific intron or 5' flanking sequence probes. In this study, we have used an intron sequence from the UBA52 gene for chromosome mapping studies. Analysis of somatic cell hybrids containing individual human chromosomes indicated that the UBA52 gene is located on chromosome 19. In situ hybridization studies confirmed the chromosomal localization but showed two peaks of hybridization: a major one over 19p13.1-p12 and a secondary one over 19q12-q13.11. Because the peak of hybridization over 19p13.1-p12 was consistently the strongest in five individuals, it is likely that this is the location of the UBA52 gene. Thus far, three of the four transcriptionally active ubiquitin genes have been assigned to separate chromosomes.

Structure and organization of the human alpha class glutathione S-transferase genes and related pseudogenes

We have isolated and characterized genomic DNA encoding several human Alpha class glutathione S-transferase genes and pseudogenes. All the genes are composed of seven exons with boundaries identical to those of the Alpha class genes in rats. The GSTA1 gene is approximately 12 kb in length and is closely flanked by other Alpha class gene sequences. The complete sequence of the 1.7-kb intergenic region between exon 7 of an upstream pseudogene and exon 1 of the GSTA1 gene has been determined. An additional gene that encodes an uncharacterized Alpha class glutathione S-transferase has been identified. The protein derived from this gene would have 19 amino acid substitutions compared with the GSTA1 isoenzyme. Several pseudogenes with single-base and/or complete exon deletions have been identified, but no reverse-transcribed pseudogenes have been detected. The occurrence of multiple genes and pseudogenes on a single fragment of cloned genomic DNA and the prior identification of a single chromosomal region (6p12) of hybridization (Board and Webb, 1987, Proc. Natl. Acad. Sci. USA 84:2377-2381) suggest that all the Alpha class genes are members of a closely linked gene family that has evolved by duplication and gene conversion events.

Factor XIII: inherited and acquired deficiency

Factor XIII (XIII), an enzyme found in plasma (present as a pro-enzyme), platelets and monocytes, is essential for normal haemostasis. It may also have a role to play in the processes of wound healing and tissue repair. Inherited XIII deficiency results in a life-long, severe bleeding diathesis which, if untreated, carries a very high risk of death in early life from intracranial bleeding. XIII is a zymogen requiring thrombin and calcium for activation. In plasma, XIII has two subunits: the 'a' subunit, which is the active enzyme, and the 'b' subunit which is a carrier protein. Activated XIII modifies the structure of clot by covalently crosslinking fibrin through an epsilon (gamma-glutamyl)lysine link. It also crosslinks other proteins, including fibronectin and alpha-2-plasmin inhibitor (alpha-2PI), into the clot through the same link. Clot modified by XIII is physically stronger, relatively more resistant to fibrinolysis and may be a more suitable medium for the ingrowth of fibroblasts. Inheritance of factor XIII is autosomal recessive. The majority of patients with the inherited defect show no XIII activity and absence of 'a' subunit protein in plasma, platelets and monocytes. At the molecular level, the defect is not a major gene rearrangement or deletion, but most likely a single point mutation which may be different in each family. Because of the severity of the bleeding diathesis, prophylaxis is desirable and has been shown to be very effective as the in vivo half-life of plasma XIII is long, and low plasma levels are sufficient for haemostasis. Acquired inhibitors have been reported in only two cases with inherited XIII deficiency. Acquired XIII deficiency has been described in a variety of diseases and bleeding has been controlled by therapy with large doses of XIII in such conditions as Henoch-Schönlein purpura, various forms of colitis, erosive gastritis and some forms of leukaemia. Large dose XIII therapy has also been used in an endeavour to promote wound healing after surgery and bone union in non-healing fractures. The use of XIII in these conditions remains controversial. Very rarely a bleeding diathesis results from the development of a specific inhibitor to XIII arising de novo, often as a complication in the course of a disease or in association with long-term drug therapy. The bleeding diathesis in these patients is difficult to treat.

Chromosomal mapping of the human Mu class glutathione S-transferases to 1p13

The chromosomal localization of the human Mu class glutathione S-transferase (GST) genes has been complicated by two factors; the total number of genes is unknown and there is a polymorphism that results from the presence or absence of the GSTM1 gene. Three human Mu class glutathione S-transferase isoenzymes, GSTM1, GSTM2, and GSTM3, have been characterized previously, and we have recently cloned and characterized GSTM4, another member of this class. Here we report that a probe derived from GSTM4 cross-hybridizes with the other three known human Mu class GST genes. In situ hybridization with the GSTM4 probe localized a major region of hybridization on chromosome band 1p13. Although there is a region of very weak hybridization on chromosome 6, these data indicate that the human Mu class gene family is largely clustered and not dispersed on different chromosomes. The identical hybridization patterns in individuals with or without the GSTM1 gene suggest that this locus is a component of the Mu class GST gene cluster.