An aspartate aminotransferase from an extremely thermophilic bacterium, Thermus thermophilus HB8

The aspartate aminotransferase gene (AspAT, EC 2.6.1.1) of an extremely thermophilic bacterium, Thermus thermophilus HB8, was cloned and sequenced, and its gene product was overproduced. The purified T. thermophilus AspAT was stable up to about 80 degrees C at neutral pH. T. thermophilus AspAT was strictly specific for acidic amino acid substrates, such as aspartate, glutamate, and the respective keto acids. The gene coding for T. thermophilus AspAT showed that it comprised 1,155 bp with a high G+C content (70 mol%), and encoded a 385-residue protein with a molecular weight of 42,050. The amino acid sequence of T. thermophilus AspAT deduced from its gene showed about 15, 46, and 29% homology with those from Escherichia coli, Bacillus sp. YM-2, and Sulfolobus solfataricus, respectively. When the amino acid sequence of T. thermophilus AspAT was compared with that of E. coli AspAT, the number of Cys was found to have decreased from 5 to 1, that of Asn from 23 to 9, that of Gln from 16 to 8, and that of Asp from 20 to 13, all of which are known to be relatively labile at high temperatures. Conversely, the number of Pro was increased from 15 to 25, Arg from 22 to 32, and Glu 27 to 37. As shown by the E. coli AspAT structure, there was a marked tendency for the extra prolyl residues to be located around the surface of the molecule. This was quite different from that in the case of RecA protein, which shows an increased number of prolyl residues in the interior of its molecule. Different strategies of different proteins as to prolyl contribution to thermostability have been suggested. Despite the high degree of conservation of active-site residues, Arg292 in E. coli AspAT, which interacts with the distal carboxylate of the substrate, was not found in T. thermophilus AspAT. Arg89 may complement the function of Arg292.

Japan's financing system for health care of the elderly

The multiplicity of Japan's health insurance system fails to achieve equality among the different insurance groups of the sharing of the financial burden. This "horizontal" inequality is effectively offset by a unique redistribution mechanism established in 1982. However, there is no reserve fund so that savings can be accumulated for the greater financial need in the future. This will inevitably lead to a heavier financial burden for the future working generation or more utilization of the personal assets of the elderly. How to achieve "vertical" or intergenerational equality in financing the cost of health care for the elderly is the biggest social policy challenge for Japan.

Characteristics of the antitumor activity of M-16 and M-18, major metabolites of a new mitomycin C derivative KW-2149, in mice

The cell growth inhibitory activity, antitumor activity and toxicity of M-16 and M-18, the major metabolites of a new mitomycin C (MMC) derivative KW-2149, in mouse and human were compared with those of KW-2149 or MMC in vitro and in vivo. The growth inhibitory activity of M-18, a symmetrical disulfide dimer, against human uterine cervix carcinoma HeLa S3 cells was almost equivalent to that of KW-2149 and their IC50 values were about 10-fold smaller than that of MMC. The activity of M-16, a methyl sulfide form, was almost equivalent to that of MMC. The cell-killing activity of MMC and M-16 was augmented in the hypoxic condition, whereas that of KW-2149 and M-18 was reduced. M-16 also exhibited almost equivalent activities to MMC in vivo in terms of many biological profiles, i.e., antitumor activity against murine P388 leukemia, ascitic or solid B16 melanoma or human lung carcinoma xenograft L-27, and bone marrow toxicity in mice. These in vitro and in vivo results indicate that the antitumor activity and toxicity of KW-2149 might not be mediated by M-16 in mice. On the other hand, M-18 exhibited almost equivalent activities to KW-2149 in these regards, suggesting the involvement of M-18 in the biological activities of KW-2149. However, the small values of the area under the curve of M-18 in mice make this unlikely. Thus the biological activities of KW-2149 in mice are not explained by the M-16 or M-18 concentration in plasma and are postulated to be manifested by KW-2149 itself.

Genetic heterogeneity of chromosome 11 associated with tumorigenicity in HeLa D98-OR cells

D98-OR is a tumorigenic subline of HeLa cells. We isolated nine subclones from D98-OR and examined their tumorigenicity in nude mice. Three, two, and four subclones were highly, weakly, and nontumorigenic, respectively. While they all contained two copies of intact chromosome 11, restriction fragment length polymorphism (RFLP) analysis revealed that the allelic composition of this chromosome differed among them. The highly tumorigenic subclones were heterozygous for the 11p and 11q loci, whereas those that were weakly or nontumorigenic were homozygous. Thus, the loss of one chromosome 11 with the duplication of another associated with the reduced tumorigenicity. Taken together with previous reports, our results indicate that a putative tumor suppressor gene on chromosome 11 controls tumorigenic expression in a gene dosage-dependent manner, and most importantly, suggested that the functional inactivation of the gene requires only a "one-hit" mutation.

Efficient production of casoxin D, a bradykinin agonist peptide derived from human casein, by Bacillus brevis

We efficiently produced a small peptide by the host-vector system using Bacillus brevis as a host. DNA encoding the physiologically functional casoxin D, composed of seven amino acids, was ligated in tandem. An expression-secretion vector containing DNA, which codes for a fusion protein of epidermal growth factor-casoxin D pentamer, was constructed. B. brevis transformed with this plasmid produced about 0.5 g/liter of the fusion protein in the culture supernatant. The fusion protein was purified with ammonium sulfate fractionation from the supernatant and digested with two kinds of proteinases. A peptide well separated by high pressure liquid chromatography was identified as biologically active casoxin D.

Hematogenous metastases of pancreatic ductal carcinoma

To investigate the heterogeneity of hematogenous metastases of pancreatic ductal carcinoma, we investigated carcinomatous spread in 130 autopsy cases. Hepatic metastases occurred most frequently, in 81 cases (62%), which may be explained by the fact that all veins draining the pancreas flow into the portal system. We closely examined the 49 cases without hepatic metastases. Sixteen patients had pulmonary metastases without hepatic metastases, whereas seven had peculiar hematogenous metastases without hepatic or pulmonary metastases. Fifteen of these 23 patients had pancreatic body carcinomas. The unusual patterns of spread might be due to (a) hepatofugal portosystemic shunting induced by splenic vein obstruction, (b) retrograde lymphatic infiltration from metastatic tracheobronchial lymph nodes, or (c) aggressive characteristics of the tumors indicated by peculiar histologic features such as pleomorphic or mucoepidermoid carcinoma, etc. Sixteen patients showed only lymph node metastases and 10 had no distant metastases. Seventeen of these 26 cases had pancreatic head carcinoma. Histologically, two patients had mucinous cystadenocarcinomas, and six had adenocarcinomas producing rich mucin. The average age of the group with no distant metastases was higher.

Reduced neck movement after operations for cervical spondylotic myelopathy

Complaints resulting from reduced neck movements were investigated in 50 patients who had operations for cervical spondylotic myelopathy. Seventy per cent had difficulty in performing 11 basic movements of daily living. Lateral bending or rotation were more difficult than flexion and extension. To look backwards was the most difficult movement. Complaints were highest among those in whom more than three levels of fusion had been carried out.

A case of pleomorphic carcinoma of the pancreas showing sequential histological change by immunohistochemical study

Pleomorphic carcinoma of the pancreas is a rare, histologically characterized pancreatic tumor with a rapid and fatal course. We report a case of a resected pleomorphic carcinoma located in the body of the pancreas in a 61-yr-old male. Histological analysis of the resected specimen revealed the coexistence of pleomorphic carcinoma with adenocarcinoma, but the recurrent tumors at autopsy 20 mo later were only of the adenocarcinomatous type. Cells in the adenocarcinomatous component showed a diffuse reactivity for CA19-9, CEA, and cytokeratin, and a focal reactivity for vimentin. In contrast, vimentin was diffusely expressed in pleomorphic lesion. Adenocarcinoma at autopsy expressed CA19-9, CEA, and cytokeratin, but not vimentin. These findings suggest that the recurrent adenocarcinoma may have developed as a consequence of sequential change in the nature of the tumor.

Angiotensin I converting enzyme inhibitory activities of various fermented foods

Angiotensin I converting enzyme (ACE, E.C. 3.4.15.1) inhibitory activity were measured with 11 kinds (31 items) of fermented foods. Strong inhibitory activity was detected in soy sauce, fish sauce, natto, nyufu, and cheese, but not in mirin, sake, or vinegar.

Mutations in the p16INK4/MTS1/CDKN2, p15INK4B/MTS2, and p18 genes in primary and metastatic lung cancer

We examined the genomic status of cyclin-dependent kinase-4 and -6 inhibitors, p16INK4,p15INK4B, and p18, in 40 primary lung cancers and 31 metastatic lung cancers. Alterations of the p16INK4 gene were detected in 6 (2 insertions and 4 homozygous deletions) of 22 metastatic non-small cell lung cancers (NSCLCs; 27%), but none were detected in 25 primary NSCLCs, 15 primary small cell lung cancers (SCLCs), or 9 metastatic SCLCs, indicating that mutation in the p16INK4 gene is a late event in NSCLC carcinogenesis. Although three intragenic mutations of the p15INK4B gene were detected in 25 primary NSCLCs (12%) and five homozygous deletions of the p15INK4B gene were detected in 22 NSCLCs (23%), no genetic alterations of the p15INK4B gene were found in primary and metastatic SCLCs. The p18 gene was wild type in these 71 lung cancers, except 1 metastatic NSCLC which showed loss of heterozygosity. We also examined alterations of these three genes and expression of p16INK4 in 21 human lung cancer cell lines. Alterations of the p16INK4 and p15INK4B genes were detected in 71% of the NSCLC cell lines (n = 14) and 50% of the NSCLC cell lines (n = 14), respectively, but there were none in the 7 SCLC cell lines studied. No p18 mutations were detected in these 21 cell lines. These results indicate that both p16INK4 and p15INK4B gene mutations are associated with tumor progression of a subset of NSCLC, but not of SCLC, and that p15INK4B mutations might also be an early event in the molecular pathogenesis of a subset of NSCLC.

Reactivity of the co-type and baa3-type cytochrome c oxidases from Pseudomonas aeruginosa with different endogenous cytochromes c

The reactivity between different cytochromes c purified from Pseudomonas aeruginosa cells grown aerobically in the absence of nitrate and isolated cytochromes co and baa3 was determined. The P. aeruginosa cytochrome co reacted most rapidly with the membrane-bound cytochrome c-551 among three c-type cytochromes analyzed, whereas the cytochrome baa3 reacted best with the membrane-bound cytochrome c-555. The results indicated that two terminal electron transfer systems are present in aerobic P. aeruginosa: one contains the cytochrome c-551 and cytochrome co, and the other contains the cytochrome c-555 and cytochrome baa3.

Effects of high pressure treatment on Ca2+ release and Ca2+ uptake of sarcoplasmic reticulum

To clarify the mechanism of pressure-induced meat tenderization or acceleration of meat conditioning, the pressure-induced morphological and biochemical changes in sarcoplasmic reticulum (SR), and Ca2+ release from SR in the rabbit skeletal muscle treated with high pressure (100-300 MPa, 5 min) were investigated in comparison with those of the SR from conditioned muscle. The destruction of the membrane structure of the SR expanded with increasing pressure applied to the muscle. Significant changes in the SDS-PAGE profile were not observed in the SR from the pressurized muscle up to 200 MPa, but a marked decrease of the ATPase protein and high-affinity Ca(2+)-binding protein were observed in the SR from the pressurized muscle at 300 MPa. The ATPase activities increased in the SR isolated from the muscle exposed to high pressure up to 200 MPa. When the muscle was pressurized at 300 MPa, the ATPase activity dropped to the same level with that of the SR from the untreated muscle. Ca2+ uptake ability of the SR vesicles measured using a fluorescent chelating reagent decreased with increasing pressure applied to the muscle. Ultrastructural studies showed that Ca2+, which was mainly localized in the SR region of the untreated fiber bundles, was translocated into myofibrillar space in the pressurized muscle. It is clear that a brief exposure of the muscle to high pressure causes considerable changes in membrane structure and biochemical function of SR as compared with those of SR in the muscle induced by conditioning.(ABSTRACT TRUNCATED AT 250 WORDS)

Anti-hypertensive substances in fermented soybean, natto

Natto is a traditional Japanese fermented food made by fermenting boiled soy beans with Bacillus natto. Its contents of inhibitors against the angiotensin converting enzyme (ACE, EC3.4.15.1) were investigated. Relatively strong inhibitory activity (IC50: 0.4 mg/ml, 11.8 inhibition units/g natto) was detected in natto extracts and the inhibitory activity observed in the viscous fraction was more potent than in the bean extract. Two groups of inhibitors in the viscous material, high and low molecular weight inhibitors, were resolved by dialysis test. The inhibitor of high molecular weight was a protein with low IC50 value (0.12 mg/ml). The two types of low molecular weight inhibitors were detected in ethanol extracts (IC50: 0.53 mg/ml and 0.95 mg/ml) and they were found to be stable over a wide range of pH and temperature up to 100 degrees C. They were different in the mode of ACE inhibition. One is competitive, and the other noncompetitive against the hydrolysis of Bz-Gly-His-Leu by ACE.

High-incidence of allelic loss at the rb gene locus in advanced human ovarian-cancer

We examined 26 ovarian cancers for allelic losses at the loci of five tumor suppressor genes, p53, DCC, RB, APC and WT1. The loss of the p53 gene was most common among these five loci (13/20, 65%). The incidence of allelic loss at the RB locus was significantly higher in advanced stage (III-IV) (6/12, 50%) than in early stage (I-II) tumors (1/14, 7%). There were no cases in which the RB gene was lost but the p53 gene was retained. These results indicate that allelic loss of the RB gene locus occurs later than that of the p53 gene and plays a role in the progression of ovarian cancer.

Differential effect of duocarmycin A and its novel derivative DU-86 on DNA strand breaks in HeLa S3 cells

Duocarmycin A (DUMA) and DU-86, a semisynthetic derivative of duocarmycins (DUMs) and a possible active form of KW-2189, both showed potent cell growth-inhibitory and cell-killing activities against human uterine cervix carcinoma HeLa S3 cells. Both drugs showed similar profiles of inhibition of macromolecular synthesis and influence on cell-cycle distribution. Namely, they inhibited [3H]thymidine uptake at lower concentrations than [3H]uridine or [3H]leucine uptake, suggesting that the inhibition of DNA synthesis is the primary site of their actions. Furthermore, they induced the accumulation of cells in early S phase. However, a significant difference was observed between these drugs in terms of DNA-fragmentation activity against HeLa S3 cells by using two independent methods, pulse-field gel electrophoresis and alkaline elution. DNA fragmentation was insignificant in the cells treated with DU-86, in contrast to the cells treated with DUMA. The analysis of DNA adducts in the cells revealed that DU-86 alkylated adenine quite selectively, while DUMA alkylated both adenine and guanine. These results suggest that the pyrrolidone ring of DUMA is responsible for its adduct formation with guanine and the subsequent DNA-fragmentation and inhibition of DNA synthesis, while DU-86 alkylated adenine and inhibited DNA synthesis through mechanisms other than DNA-fragmentation.

Overexpression of human cyclin D1 reduces the transforming growth factor beta (TGF-beta) type II receptor and growth inhibition by TGF-beta 1 in an immortalized human esophageal epithelial cell line

Cyclin D1 has been implicated in G1 cell cycle progression and is frequently amplified, overtranscribed, and oversynthesized in human tumors, including esophageal carcinomas. To further address the role of cyclin D1 in cell cycle control and tumorigenesis, we have stably transfected the human cyclin D1 in the nontumorigenic esophageal epithelial cell line HET-1A. These transfected cells, which express increased amounts of cyclin D1, have enhanced colony-forming efficiency and saturation density and are resistant to growth inhibition by TGF-beta 1 compared with the parental cell line or a control vector cell clone. The clones which express increased amounts of cyclin D1 exhibited a decrease in the amount of TGF-beta type II receptor, indicating a plausible mechanism for their diminished response to TGF-beta 1. Therefore, deregulated expression of the cyclin D1 gene can modulate the negative growth factor pathway of TGF-beta 1 and may disturb the control of epithelial cell proliferation in esophageal carcinogenesis.

Mutations and altered expression of p16INK4 in human cancer

Cell cycle arrest at the G1 checkpoint allows completion of critical macromolecular events prior to S phase. Regulators of the G1 checkpoint include an inhibitor of cyclin-dependent kinase, p16INK4; two tumor-suppressor proteins, p53 and RB (the product of the retinoblastoma-susceptibility gene); and cyclin D1. Neither p16INK4 nor the RB protein was detected in 28 of 29 tumor cell lines from human lung, esophagus, liver, colon, and pancreas. The presence of p16INK4 protein is inversely correlated with detectable RB or cyclin D1 proteins and is not correlated with p53 mutations. Homozygous deletions of p16INK4 were detected in several cell lines, but intragenic mutations of this gene were unusual in either cell lines or primary tumors. Transfection of the p16INK4 cDNA expression vector into carcinoma cells inhibits their colony-forming efficiency and the p16INK4 expressing cells are selected against with continued passage in vitro. These results are consistent with the hypothesis that p16INK4 is a tumor-suppressor protein and that genetic and epigenetic abnormalities in genes controlling the G1 checkpoint can lead to both escape from senescence and cancer formation.

Intraoperative radiation therapy for pancreatic carcinoma. The choice of treatment modality

Ninety patients with carcinoma of the pancreas treated between 1976 and 1990 were reviewed retrospectively. Intraoperative radiation therapy (IORT) in combination with external beam radiation therapy (EBRT) for localized but unresectable tumors (n = 29) prolonged survival significantly more than IORT alone (n = 16) (p < 0.01); it seems EBRT enhanced or contributed to the better results obtained with IORT plus EBRT. Moreover, IORT, alone or in combination, relieved pain. Adjuvant IORT for residual tumors (n = 20) might not effectively prolong survival, because the difference in survival rate between noncurative resection plus IORT and nonresection plus IORT in combination with EBRT was not significant. Curative tumor resection of stage III disease in combination with IORT (n = 9) resulted in significantly longer survival as compared with curative tumor resection alone (n = 8) (p < 0.05). It may be advisable to administer IORT in combination with EBRT to patients with advanced pancreatic carcinoma, avoiding aggressive tumor resection, when curative tumor resection cannot be performed.

X-ray crystallographic study of pyridoxal 5'-phosphate-type aspartate aminotransferases from Escherichia coli in open and closed form

We determined the three-dimensional structures of aspartate aminotransferase (AspAT) from Escherichia coli and its complex with inhibitor (2-methyl-L-aspartate) at 1.8A resolution. This enzyme reversibly catalyzes the transamination reaction and is a dimer of two identical subunits. Each subunit has 396 amino acid residues and one pyridoxal 5'-phosphate as a cofactor, and is divided into two domains, one large and the other small. Upon binding of the inhibitor, the small domain rotates by 5 degrees toward the large domain to close the active site. This domain movement is caused mainly by small but important main-chain conformational changes in the residues located over the domain interface of the small domain. In chicken mitochondrial AspAT, the domain movement was larger, with a rotational angle of 13 degrees. By comparison of these two structures, the difference in the rotational angles was found to be caused by the larger opening of the domain in the open form of chicken mitochondrial AspAT. Although the overall structures of these two enzymes were almost identical, the surface area of the domain interface in the E. coli enzyme was larger than that in mitochondrial AspAT, suggesting that the structure of the domain interface is responsible for the degree of movement of the small domain.

In vivo wear pattern of experimental composite resins containing different filler components

Using a scanning electron microscope (SEM) and an electron probe surface roughness analyzer (ERA), we conducted the in vivo study of the effect of various filler components on the wear of composite resins. Experimental light-cured composite resins were prepared employing three different filler components; -(1) Silica type, (2) Strontium type, and (3) Barium type. The filler content for all three types was 80 wt%, with the mean particlesize being 2.6 microns in diameter. The resin monomers consisted of 40 wt% Bis-GMA, 40 wt% TEGDMA and 20 wt% UTMA. These materials were placed in 2 mm diameter cylindrical cavities located in the OCA (occlusal contact area) or the CFA (contact free area) of cast crowns temporarily set in a volunteer patient's mouth. The crowns were removed at monthly intervals for longitudinal SEM observation. After two months, worn surfaces were also analyzed by ERA. The result showed that the wear patterns of the composites were characterized by the filler components, especially in the OCA.

Interactions between neutral endopeptidase (EC 3.4.24.11) and the substance P (NK1) receptor expressed in mammalian cells

Interactions between neutral endopeptidase-24.11 (NEP) and the substance P receptor (SPR; NK1) were investigated by examining substance P (SP) degradation, SP binding and SP-induced Ca2+ mobilization in epithelial cells transfected with cDNA encoding the rat SPR and rat NEP. Expression of NEP accelerated the degradation of SP by intact epithelial cells and by membrane preparations, and degradation was reduced by the NEP inhibitor thiorphan. In cells expressing SPR alone, specific 125I-SP binding after 20 min incubation at 37 degrees C was 92.2 +/- 3.1% of maximal binding and was unaffected by thiorphan. Coexpression of NEP in the same cells as the SPR markedly reduced SP binding to 13.9 +/- 0.5% of maximal, and binding was increased to 82.7 +/- 2.4% of maximal with thiorphan. Coexpression of NEP in the same cells as the SPR also reduced to undetectable the increase in intracellular Ca2+ in response to low concentrations of SP (0.3 and 0.5 nM), and significantly reduced the response to higher concentrations (1 and 3 nM). The Ca2+ response was restored to control values by inhibition of NEP with thiorphan. In contrast, SP binding and SP-induced Ca2+ mobilization were only slightly reduced when cells expressing SPR alone were mixed with a 3- to 24-fold excess of cells expressing NEP alone. Therefore, in this system, NEP markedly down-regulates SP binding and SP-induced Ca2+ mobilization only when coexpressed in the same cells as the SPR.

Characteristics of antitumor activity of KW-2189, a novel water-soluble derivative of duocarmycin, against murine and human tumors

Methyl(1S)-1-bromomethyl-7-methyl-5-[(4-methylpiperazinyl)-carb onyloxy]- 3-[(5,6,7-trimethoxy-2-indolyl)-carbonyl]-1,2-dihydro-3H-pyrolo[3, 2-e] indole-8-carboxylate hydrobromide (KW-2189), a novel derivative of duocarmycin B2, was selected for extensive evaluation based on its improved antitumor activity, water solubility, and stability in the culture medium, as compared with duocarmycin B2. Although the in vitro cell growth-inhibitory activity of KW-2189 was less potent than that of duocarmycin B2, it significantly inhibited the growth of five murine solid tumors including Colon 26 adenocarcinoma, Colon 38 adenocarcinoma, and B16 melanoma in vivo. KW-2189 was also effective against murine P388 leukemia and L1210 leukemia not only by local administration (i.p.-i.p. system), but also by systemic administration (i.p.-i.v. or i.v.-i.v. system). The most remarkable feature of KW-2189 was its efficacy against various human xenografts, which was observed in 14 tumors among 16 tested tumors including drug-insensitive tumors by single i.v. administration. Tumor regression was observed in mice bearing LC-6 lung, St-4 and St-40 stomach, Li-7 liver, PAN-2 pancreas, and MX-1 breast carcinomas. In many cases, the activities of KW-2189 were more than those of clinically active agents, mitomycin C, Adriamycin, cisplatin, and cyclophosphamide. Delayed lethal toxicity, which was reported in mice treated with CC-1065 whose structure was similar to KW-2189, was not observed in mice treated with KW-2189. KW-2189 inhibited DNA synthesis more significantly than RNA or protein synthesis, although DNA strand breaks were not observed. KW-2189 was activated by porcine liver esterase, mouse liver homogenate or Hep G2 homogenate, and DU-86-DNA adducts were detected in KW-2189-treated HeLa S3 cells, suggesting that KW-2189 was converted to DU-86 in the cells. These results indicate that KW-2189 is an interesting candidate for further development as a novel antitumor agent.

Viscoelasticity of hyaluronic acid with different molecular weights

Storage and loss moduli of hyaluronic acid solutions with different molecular weights were observed as a function of frequency in the presence of sugars and salts. The hyaluronic acid solutions of higher molecular weight (Mw > 17 x 10(6)) fractions showed entanglement, whereas lower molecular weight fractions did not. For inducing the entanglement of molecular chains of hyaluronic acid, increasing the molecular weight of hyaluronic acid was more effective than increasing the concentration of the lower molecular weight fractions (Mw = 7. 8 x 10(5). Glucose, fructose, galactose, and sucrose increased both storage and loss moduli, while NaCl, KCl and CaCl2 decreased both moduli. It is suggested that sugars create hydrogen bonds and strengthen the transient network. Cations shield the electrostatic repulsion of hyaluronic acid molecules, and the polymer chains are shrunk into compact coils from expanded stiffened coils.

Construction of aminotransferase chimeras and analysis of their substrate specificity

Escherichia coli aspartate aminotransferase (AspAT) and E. coli aromatic amino acid aminotransferase (AroAT) have almost identical and high activities toward acidic amino acid substrates. AroAT also has high activity toward aromatic amino acid substrates. The two proteins have 44% amino acid sequence homology. In order to study the mechanism responsible for the different substrate specificities of these aminotransferases, chimeric enzymes of AspAT and AroAT were constructed using homologous recombination in E. coli cells. Five chimeric enzymes were obtained, even though the nucleotide sequence homology between the two parent enzymes was as low as about 50%. The yields of the legitimate chimeric genes were related to the lengths of the homologous region between the two parent genes. Homologous recombination occurred in the region where more than eight nucleotides out of ten were identical. The substrate specificity of the chimeric enzymes suggest that not only the amino acid residues in the active site but also those distant from the active site contribute to the substrate specificity of the parental aminotransferases.