The diagnostic accuracy/efficacy of MRI in differentiating hepatic hemangiomas from metastatic colorectal/breast carcinoma: a multiple reader ROC analysis using a jackknife technique

PURPOSE

Our purpose was to determine the diagnostic accuracy efficacy of a simple MR technique in differentiating hepatic hemangiomas from colorectal or breast metastases using a multiple reader method.

METHOD

Thirty-seven cases with confirmed hepatic hemangiomas and 115 with confirmed hepatic metastases (colon primary, n = 86; breast primary, n = 29) evaluated with MRI at 1.5 T were retrospectively collected. A single lesion in a single slice from each patient was randomly selected; the images were masked and then were interpreted in random order by five separate readers blinded to the diagnosis using a five point diagnostic scale (from definite hemangioma to definite metastasis). Morphologic characteristics of lesion margin, signal intensity relative to other structures, and internal architecture (homogeneous versus heterogeneous) were also assessed independently of the five point diagnostic scale. Three of the readers had > 8 years of experience, while the other two had 1 and 3 years. The diagnostic scale results were subjected to receiver operating characteristic (ROC) analysis using a jackknife method. kappa-Statistics were applied to assess interreader agreement in the morphologic characteristics. A logistic regression model was used to determine which characteristics predicted pathology and reader diagnosis.

RESULTS

ROC analysis showed the average area under the curve over all readers was (0.91 (0.89-0.93 95% confidence interval) (p < 0.0001). An analysis of variance showed no significant difference between the areas under the curves of each reader (p = 0.6433). When the definite and probable categories for hemangioma and metastasis were combined, the sensitivity/specificity for the diagnosis of hemangioma ranged from 57 to 73%/91 to 97%. The positive/negative predictive value ranged from 72 to 84%/87 to 91%. For the morphologic assessment, there was significant agreement between the readers (p < 0.0001-0.0037). A sharp margin and lesion signal equal to or greater than CSF predicted the presence of a hemangioma (p = 0.0148 and p < 0.0001, respectively). A sharp margin, lesion signal equal to or greater than CSF, and a homogeneous internal architecture all predicted the reader diagnosis of definitely or probably hemangioma.

CONCLUSION

For multiple readers, T2-weighted SE MRI alone is a very specific method for distinguishing hemangiomas from metastatic colon or breast carcinoma. Morphologic characteristics of a sharp margin and a high signal predict the presence of a hemangioma. Last, reader experience does not appear to have a significant effect on the specificity.

Structures important in NAD(P)(H) specificity for mammalian retinol and 11-Cis-retinol dehydrogenases

Mammalian retinol and 11-cis retinol dehydrogenases catalyze the formation of retinaldehyde from retinol and 11-cis retinaldehyde from 11-cis retinol, respectively. Although their amino acid sequences are 54% identical, these enzymes have different cofactor specificities: rat retinol dehydrogenase uses NADP+, while cow 11-cis retinol dehydrogenase uses NAD+. We used their close sequence similarity and the availability of 3D structures of their homologs to construct a 3D model of the two retinol dehydrogenases to investigate the determinants for cofactor specificity. The structure of rat retinol dehydrogenase shows that lysine-64 is important in stabilizing binding of 2'-phosphate on NADP+ in two ways: lysine's positively charged side chain has a coulombic attraction to the 2'-phosphate and partially compensates for the negative charge of aspartic acid-38. Cow 11-cis retinol dehydrogenase has threonine-61 at the position homologous to lysine-64. Threonine-61 does not have a stabilizing coulombic interaction with NADP+, nor can threonine-61 counteract the repulsive interaction between NADP+ and aspartic acid-37 in 11-cis retinol dehydrogenase. This suggests that aspartic acid-37 and threonine-61 are important in the specificity of 11-cis retinol dehydrogenase for NAD+.

Regeneration of a transplanted liver after right hepatic lobectomy

Liver regeneration has been described after heterotopic liver transplantation, small-for-size orthotopic liver transplantation and reduced-size liver transplantation. In this report, we document the regenerative response of a whole liver transplant to major resection for the first time. A right hepatic lobectomy for liver ischemia was performed in a 30-year-old female after transplantation for autoimmune disease of the liver. Volumetric analysis of computed tomography (CT) scans revealed a preoperative liver volume of 1,961 mL, whereas analysis of the 6-week posthepatectomy CT scan showed a volume of 1,820 mL. Factors influencing regeneration in the setting of a liver transplant include rejection, ischemia/thrombosis, infection, or cyclosporine hepatotrophic/hepatotoxic effects. These factors, balanced with the intrinsic ability of hepatocytes to achieve a standard liver volume, determine the extent of regeneration.

Unusual evolution of 11beta- and 17beta-hydroxysteroid and retinol dehydrogenases

11beta-hydroxysteroid dehydrogenases regulate glucocorticoid concentrations and 17beta-hydroxysteroid dehydrogenases regulate estrogen and androgen concentrations in mammals. Phylogenetic analysis of the sequences from two 11beta-hydroxysteroid dehydrogenases and four mammalian 17beta-hydroxysteroid dehydrogenases indicates unusual evolution in these enzymes. Type 1 11beta- and 17beta-hydroxysteroid dehydrogenases are on the same branch; Type 2 enzymes cluster on another branch with beta-hydroxybutyrate dehydrogenase,11-cis-retinol dehydrogenase and retinol dehydrogenase; Type 3 17beta-hydroxysteroid dehydrogenase is on a third branch; while the pig dehydrogenase clusters with a yeast multifunctional enzyme on a fourth branch. Pig 17beta-hydroxysteroid dehydrogenase appears to have evolved independently from the other three 17beta-hydroxysteroid dehydrogenases; in which case, the evolution of 17beta-hydroxysteroid dehydrogenase activity is an example of functional convergence. The phylogeny also suggests that independent evolution of specificity toward C11 substituents on glucocorticoids and C17 substituents on androgens and estrogens has occurred in Types 1 and 2 11beta- and 17beta-hydroxysteroid dehydrogenases.

Structures stabilizing the dimer interface on human 11 beta-hydroxysteroid dehydrogenase types 1 and 2 and human 15-hydroxyprostaglandin dehydrogenase and their homologs

Human 11 beta-hydroxysteroid dehydrogenase-types 1 and 2 and human 15-hydroxyprostaglandin dehydrogenase belong to a large family of oxidoreductases that includes human dihydropteridine reductase and Streptomyces hydrogenans 20 beta-hydroxysteroid dehydrogenase, for which 3D structures are available. Almost all of these enzymes are either dimers or tetramers. The dimer interface of rat dihydropteridine reductase consists of alpha-helices E and F from each monomer arranged in a four alpha-helix bundle [Varughese et al. (1992) Proc. Natl. Acad. Sci. USA 89, 6080-6084]. Alpha-helix F contains tyrosine-146 and lysine-150, residues that are highly conserved in this protein superfamily and have been proposed to be at the catalytic site. We have examined the dimer interface between alpha-helix F in human and rat dihydropteridine reductase and Streptomyces hydrogenans 20 beta-hydroxysteroid dehydrogenase as well as modeled 3D structures of steroid and prostaglandin dehydrogenases and homologs for stabilizing interactions. We find a site in the middle of alpha-helix F that stabilizes the dimer. This anchor is adjacent to conserved lysine on alpha-helix F. Our analysis suggests that sequence variation in the anchor may be important in substrate specificity.

Structures important in mammalian 11 beta- and 17 beta-hydroxysteroid dehydrogenases

We have used the X-ray crystallographic structures of rat and human dihydropteridine reductase and Streptomyces hydrogenans 20 beta-hydroxysteroid dehydrogenase to model parts of the 3-dimensional structure of human 11 beta- and 17 beta-hydroxysteroid dehydrogenases. We use this information along with previous results from studies of Drosophila alcohol dehydrogenase mutants to analyze the structures in binding sites for NAD(H) and NADP(H) in 11 beta-hydroxysteroid dehydrogenase-types 1 and 2. We also examine the structure of an alpha-helix at catalytic site of 17 beta-hydroxysteroid dehydrogenase-types 1, 2, 3, and 4. This alpha-helix contains a highly conserved tyrosine and lysine. Adjacent to the carboxyl side of this lysine is a site proposed to be important in subunit association. We find that 11 beta- and 17 beta-hydroxysteroid dehydrogenases-type 1 have the same residues at the "anchor site" and conserve other stabilizing features, despite only 20% sequence identity between their entire sequences. Similar conservation of stabilizing structures is found in the 11 beta- and 17 beta-hydroxysteroid dehydrogenases-type 2. We suggest that interactions of the dimerization surface of alpha-helix F with proteins or membranes may be important in regulating activity of hydroxysteroid dehydrogenases.

The current role of percutaneous biopsy in the evaluation of renal masses

Percutaneous biopsy of intraabdominal masses is a safe and accurate means of obtaining a tissue diagnosis without surgical exploration. It is often sufficient to determine treatment plans and in many instances, obviates the need for surgery. Percutaneous biopsy of renal masses has traditionally had a limited role in the United States. However, with a recent increase in the detection of small renal masses because of the widespread use of abdominal computed tomography (CT), there is an increasing role for percutaneous biopsy in the management of renal masses. Percutaneous biopsy of renal masses is indicated to differentiate between a primary renal cell carcinoma and metastatic disease in patients with a known extrarenal primary. Percutaneous biopsy is also indicated to establish a diagnosis of renal lymphoma and abscess. Complications of percutaneous biopsy include bleeding, pneumothorax, and tumor seeding along the needle tract; fortunately, these complications are uncommon. We will review the indications, techniques, complications, sensitivity, and accuracy of CT and ultrasound-guided biopsy of renal masses. Urologists and radiologists should both be familiar with the indications and contraindications of percutaneous biopsy to insure the appropriate management of renal masses.

Enoyl-acyl-carrier-protein reductase and Mycobacterium tuberculosis InhA do not conserve the Tyr-Xaa-Xaa-Xaa-Lys motif in mammalian 11 beta- and 17 beta-hydroxysteroid dehydrogenases and Drosophila alcohol dehydrogenase

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Licorice-derived compounds inhibit linoleic acid (C:18:2 omega 6) desaturation in soybean chloroplasts

Although glycyrrhizic acid, a major constituent of licorice root, has important pharmacological effects in humans, the biological activity of glycyrrhizic acid and its aglycone glycyrrhetinic acid in plants is unknown. Here we report that these licorice-derived compounds and the analog carbenoxolone inhibit desaturation of linoleic acid (C18:2 omega 6) in soybean chloroplasts using monogalactosyldiacylglycerol and phosphatidylcholine substrates in an in vitro assay for desaturase activity. At 10 nM glycyrrhetinic acid, there is significant inhibition of desaturation of linoleic acid suggesting that licorice-derived compounds could prove useful in investigating biochemical pathways of linoleic acid desaturation in plant chloroplasts and plant desaturase regulation, which has application in modification of plant response to environmental stress, as well as optimization of oil seed composition.

Amino acids important in enzyme activity and dimer stability for Drosophila alcohol dehydrogenase

We have determined the nucleotide sequences of eight ethyl methanesulphonate-induced mutants in Drosophila alcohol dehydrogenase (ADH), of which six were previously characterized by Hollocher and Place [(1988) Genetics 116, 253-263 and 265-274]. Four of these ADH mutants contain a single amino acid change: glycine-17 to arginine, glycine-93 to glutamic acid, alanine-159 to threonine, and glycine-184 to aspartic acid. Although these mutants are inactive, three mutants (Gly17Arg, Gly93Glu and Gly184Asp) form stable homodimers, as well as heterodimers with wild-type ADH, in which the wild-type ADH subunit retains full enzyme activity [Hollocher and Place (1988) Genetics 116, 265-274]. Interestingly, the Ala159Thr mutant does not form either stable homodimers or heterodimers with wild-type ADH, suggesting that alanine-159 is important in stabilizing ADH dimers. The mutations were analysed in terms of a three-dimensional model of ADH using bacterial 20 beta-hydroxysteroid dehydrogenase and rat dihydropteridine reductase as templates. The model indicates that mutations in glycine-17 and glycine-93 affect the binding of NAD+. It also shows that alanine-159 is part of a hydrophobic anchor on the dimer interface of ADH. Replacement of alanine-159 with threonine, which has a larger side chain and can hydrogen bond with water, is likely to reduce the strength of the hydrophobic interaction. The three-dimensional model shows that glycine-184 is close to the substrate binding site. Replacement of glycine-184 with aspartic acid is likely to alter the position of threonine-186, which we propose hydrogen bonds to the carboxamide moiety of NAD+. Also, the negative charge on the aspartic acid side chain may interact with the substrate and/or residues in the substrate binding site. These mutations provide information about ADH catalysis and the stability of dimers, which may also be useful in understanding homologous dehydrogenases, which include the human 17 beta-hydroxysteroid, 11 beta-hydroxysteroid and 15-hydroxyprostaglandin dehydrogenases.

Midgut carcinoid tumors: CT findings and biochemical profiles

OBJECTIVE

Our goal was to describe the abdominal CT findings in 52 patients with midgut carcinoid tumors and correlate these findings with their biochemical profiles.

MATERIALS AND METHODS

Abdominal/pelvic CTs of 52 patients with midgut carcinoid tumors were reviewed retrospectively for the presence of liver metastases, mesenteric and peritoneal disease, bowel changes, lymphadenopathy, and the presence of the primary tumor. Logistic regression models were used to evaluate the association between these findings and the serum and platelet serotonin and urine 5-hydroxyindolacetic acid levels.

RESULTS

The most common finding was hepatic metastases (34/52). Nonspecific mesenteric soft tissue stranding was present in 26 of 52 and a discrete mesenteric mass was present in 25 of 52. These masses had linear, radiating soft tissue spokes in 16 of 25 and contained calcification in 10 of 25. Retroperitoneal and mesenteric lymphadenopathy was present in 14 of 52 and 11 of 52 cases, respectively. Carcinomatosis was present in 11 of 52. Bowel wall thickening was seen in 9 of 52. Six patients had a small bowel obstruction. Elevated serum serotonin, platelet serotonin, and urine 5-hydroxyindolacetic acid levels were significantly associated with the presence of liver metastases (p = 0.0032, 0.0098, and 0.0450, respectively). Elevated platelet serotonin levels were also significantly associated with the presence of a mesenteric mass (p = 0.0101).

CONCLUSION

In our population, the most common findings of a midgut carcinoid tumor are liver metastases, nonspecific mesenteric soft tissue changes, a discrete mesenteric mass with radiating soft tissue spokes, often containing calcification, and lymphadenopathy. As expected, liver metastases correlate strongly with the presence of elevated biochemical levels. A new observation is the correlation of elevated platelet serotonin levels and mesenteric masses. We hypothesize that platelet serotonin may be the factor that stimulates stromal cells to produce mesenteric fibrosis and mass formation.

Endocrine activity of plant-derived compounds: an evolutionary perspective

Although plants have long been known to have important pharmacological effects in humans, the mechanism by which plant-derived compounds act in humans is still being elucidated. Two important pathways for the biological actions of plant-derived compounds involved binding either to hormone receptors or to enzymes that metabolize hormones. What are the origins of this interaction between plant-derived compounds and animals? And what insights can we gain from investigating this question? Some answers come from recent sequence analyses, revealing that 17 beta-hydroxysteroid dehydrogenase, which regulates estrogen and androgen levels in humans, and 15-hydroxyprostaglandin dehydrogenase, which regulates prostaglandin E2 and F2 alpha levels in humans, have a common ancestor with proteins in rhizobia that are important in forming nitrogen-fixing nodules in legume roots, and 3 beta-hydroxysteroid dehydrogenase, which regulates progestin and androgen levels in humans, has a common ancestor with enzymes important in the synthesis of anthocyanins. This evolutionary kinship, when combined with the structural similarities between flavonoids, licorice-derived compounds, and steroid hormones, provides another perspective on the hormone-like activity of flavonoids and other plant-derived compounds in humans: some of the hormone-like activity of plant-derived compounds is due to binding to steroid and prostaglandin dehydrogenases.

Computed tomography and magnetic resonance imaging in the assessment of pancreatic disease

Significant technical advances have occurred in both computed tomography (CT) and magnetic resonance (MR) imaging that have resulted in improved visualization of the pancreas by both techniques. In this article we will consider the advances in both modalities and consider the relative roles of each in imaging the spectrum of pancreatic pathology.

Adding a positive charge at residue 46 of Drosophila alcohol dehydrogenase increases cofactor specificity for NADP+

We previously reported that the D39N mutant of Drosophila alcohol dehydrogenase (ADH), in which Asp-39 is replaced with asparagine, has a 60-fold increase in affinity for NADP+ and a 1.5-fold increase in kcat compared to wild-type ADH [Chen et al. (1991) Eur. J. Biochem. 202, 263-267] and proposed that this part of ADH is close to the 2'-phosphate on the ribose moiety of NADP+. Here we report the effect of replacing Ala-46 with an argine residue, and A46R mutant, on binding of NADP+ to ADH and its catalytic efficiency with the NADP+ cofactor, and a modeling of the three-dimensional structure of the NAD(+)-binding region of ADH. The A46R mutant has a 2.5-fold lower Km(app)NADP+ and a 3-fold higher kcat with NADP+ compared to wild-type ADH; binding of NAD+ to the mutant was unchanged and kcat with NAD+ was lowered by about 30%. For the A46R mutant, the ratio of kcat/Km of NAD+ to NADP+ is 85, over ten-fold lower than that for wild-type ADH. Our model of the 3D structure of the NAD(+)-binding region of ADH shows that Ala-46 is over 10 A from the ribose moiety of NAD+, which would suggest that there is little interaction between this residue and NAD+ and explain why its mutation to arginine has little effect on NAD+ binding. However, the positive charge at residue 46 can neutralize some of the coulombic repulsion between Asp-39 and the 2'-phosphate on the ribose moiety of NADP+, which would increase its affinity for the A46R mutant. We also constructed a double mutant, D39N/A46R mutant, which we find has a 30-fold lower Km(app)NADP+ and 8-fold higher kcat with NADP+ as a cofactor compared to wild-type ADH; binding of NAD+ to this double mutant was lowered by 5-fold and kcat was increased by 1.5-fold. As a result, kcat/Km for the double mutant was the same for NAD+ and NADP+. The principle effect of the two mutations in ADH is to alter its affinity for the nucleotide cofactor; kcat decreases slightly in A46R with NAD+ and remains unchanged or increases in the other mutants.

CT of the liver in patients with metastatic breast carcinoma treated by chemotherapy: findings simulating cirrhosis

OBJECTIVE

Although the hepatotoxic effects of systemic chemotherapy are well known, CT findings in the liver after systemic chemotherapy have received little attention in the literature. In some patients with breast carcinoma metastatic to the liver who have received chemotherapy, a morphologic pattern develops similar to that associated with cirrhosis. This pattern is characterized by a lobular hepatic contour, segmental volume loss, and enlargement of the caudate lobe. The purpose of this study was to describe the CT appearance of this pseudocirrhosis and to correlate it with pathologic findings.

MATERIALS AND METHODS

We reviewed 65 CT examinations of 22 patients with stage IV breast carcinoma with hepatic metastases, who were receiving systemic chemotherapy and for whom abdominal CT scans showed pseudocirrhosis. Nineteen of 22 patients had follow-up CT scans at intervals ranging from 1 to 15 months. Criteria for the diagnosis of pseudocirrhosis included a lobular hepatic contour, segmental volume loss, and enlargement of the caudate lobe. CT findings were correlated with pathologic findings in seven patients.

RESULTS

In all patients, CT scans showed retraction of the capsular surface of the liver (15 diffuse, seven focal) with a lobular margin, a finding also seen in advanced cirrhosis. The retraction occurred at the site of subjacent metastases. Findings evolved over 1-3 months. Six of seven patients had pathologic findings suggestive of nodular regenerative hyperplasia. No patients had pathologic evidence of cirrhosis.

CONCLUSION

In patients undergoing systemic chemotherapy for breast cancer metastatic to the liver, a pattern may develop that mimics the CT appearance of hepatic cirrhosis. Pathologic findings suggest nodular regenerative hyperplasia as a possible cause.

Detection of focal hepatic masses: STIR MR vs. CT during arterial portography

OBJECTIVE

Our goal was to compare short tau inversion recovery MRI (STIR) to CT during arterial portography (CTAP) in the detection of hepatic lesions.

MATERIALS AND METHODS

Over a 24 month period, 40 patients evaluated for possible hepatic resection underwent T1, T2, and STIR MRI and CTAP. These studies were randomly ordered and interpreted by two independent readers. The total number of lesions and number of lesions within each hepatic segment were analyzed using McNemar's test.

RESULTS

Readers 1 and 2 detected 95 and 91 lesions by CTAP and 78 and 83 by STIR, respectively, which was not significantly different (p = 0.1, reader 1; p = 0.4, reader 2). For both readers CTAP and STIR MRI detected more lesions than either T2 or T1 MRI (p < 0.01). In the medial segment, readers 1 and 2 detected 18 and 17 lesions by CTAP and 7 and 9 by STIR, respectively (p = 0.01, reader 1; p = 0.035, reader 2).

CONCLUSION

Short tau inversion recovery MRI and CTAP are similar in total lesion detection except in the medial segment where lesions were more frequently identified by CTAP than STIR MRI.

Myxococcus xanthus C-factor, a morphogenetic paracrine signal, is similar to Escherichia coli 3-oxoacyl-[acyl-carrier-protein] reductase and human 17 beta-hydroxysteroid dehydrogenase

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Hepatic adenoma: MR characteristics and correlation with pathologic findings

OBJECTIVE

The purpose of this study was to describe the MR appearance of hepatic adenomas and correlate the MR imaging features with pathologic findings.

MATERIALS AND METHODS

MR examinations were performed in 14 patients with 66 hepatic adenomas. The diagnosis of hepatic adenoma was proved pathologically in nine patients (22 lesions). In five other patients (44 lesions), who had type I glycogen storage disease and were known to be at risk for hepatic adenomas, the diagnosis was established by repeated sonographic examinations that showed stability, reduction, or resolution of hepatic tumors. T1- and T2-weighted spin-echo MR images obtained at 1.5 T were retrospectively reviewed for the signal intensity of the lesions relative to liver, the signal pattern, the presence of a capsule, and the presence of hemorrhage. Histopathologic specimens (22 lesions) were reviewed for fat content (graded 0-3), the presence of a capsule, and the presence of hemorrhage.

RESULTS

On T1-weighted images, 51 (77%) of 66 lesions were hyperintense, 11 (17%) were hypointense, and four (6%) were isointense with respect to liver. On T2-weighted images, 49 (74%) of 66 lesions were hyperintense, 12 (18%) were isointense, and five (8%) were hypointense. Sixty-one (92%) of 66 lesions were heterogeneous. Eleven (17%) of 66 lesions were hemorrhagic. Of the 22 lesions reviewed histopathologically, 17 were hyperintense on T1-weighted images; 15 of these had a fat content of grade 2 or 3 and two had hemorrhage. All 15 lesions that had a fat content of grade 2 or 3 were hyperintense on T1-weighted images.

CONCLUSION

Hepatic adenomas have a variable MR appearance but most often are hyperintense with respect to liver on T1- and T2-weighted images. The high signal intensity often relates to the increased fat content of these lesions.

Protochlorophyllide reductase is homologous to human carbonyl reductase and pig 20 beta-hydroxysteroid dehydrogenase

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Development of an optical formaldehyde sensor based on the use of immobilized pararosaniline

The colorimetric indicator pararosaniline has been immobilized onto the cation-exchange resins Amberlite IRC-50, Dowex 50W-X8 and cellulose phosphate by electrostatic bonding. The reflectance of each reagent phase was measured using a bifurcated fibre-optic system and a flow cell. Pararosaniline immobilized on cellulose phosphate was found to respond to formaldehyde without requiring the addition of sulfite to develop the purple chromogen. This immobilized system demonstrated a linear response to 50-2500 micrograms of formaldehyde and had a correlation coefficient of 0.9979. Acetaldehyde and butyraldehyde did not produce any interference. However, exposure to the unsaturated aldehydes, acrolein and crotonaldehyde, gave rise to responses that were much greater than that observed with formaldehyde.

Sequence analysis of steroid- and prostaglandin-metabolizing enzymes: application to understanding catalysis

Amino acid sequence comparisons have revealed that mammalian 11 beta-hydroxysteroid and 17 beta-hydroxysteroid dehydrogenases and bacterial 3 alpha, 20 beta- and 3 beta-hydroxysteroid dehydrogenases are homologs; that is, these enzymes are descended from a common ancestor. These steroid dehydrogenases are also homologous to human 15-hydroxyprostaglandin dehydrogenase and to proteins found in Rhizobia, bacteria that form nitrogen-fixing nodules in the roots of legumes. We constructed a multiple sequence alignment of these proteins, which, when combined with the recently determined tertiary structure of Streptomyces hydrogenans 3 alpha, 20 beta-hydroxysteroid dehydrogenase and a homologous enzyme, rat dihydropteridine reductase, identifies segments and residues that are likely to be structurally important in the functioning of these enzymes especially regarding specificity for NADPH and NADH.