No abstract

The t(9;22) chromosomal translocation is found in almost all patients with chronic myelogenous leukemia. The resultant Bcr-Abl fusion gene expresses a chimeric fusion protein p210 bcr-abl with increased tyrosine kinase activity. Hematopoietic progenitors isolated from chronic myelogenous leukemia patients in the chronic phase contain constitutively tyrosine-phosphorylated p62 dok protein. p62 dok associates with the Ras GTPase-activating protein (RasGAP), but only when p62 dok is tyrosine phosphorylated. Here we have investigated the interaction between p62 dok and RasGAP and the consequences of p62 dok tyrosine phosphorylation on the activity of RasGAP. We have found that p62 dok is directly tyrosine phosphorylated by p210 bcr-abl , and the sites of phosphorylation are located in the C-terminal half of the p62 dok molecule. We have identified five tyrosine residues that are involved in in vitro RasGAP binding and have found that tyrosine-phosphorylated p62 dok inhibits RasGAP activity. Our results suggest that p210 bcr-abl might lead to the activation of the Ras signaling pathway by inhibiting a key down-regulator of Ras signaling.

The DNA strand-breaking activity of some dihydropyrazine derivatives, 2,3-dihydro-5,6-dimethylpyrazine (3), 2,3-dihydro-2,5,6-trimethylpyrazine (4), 2,3-dihydro-2,2,5,6-tetramethylpyrazine (5), trans-2,3-dimethyl-5,6,7,8,9, 10-hexahydroquinoxaline (6), its cis-compound (7) and the mixture of 6 and 7 (8) was tested by agarose gel electrophoresis using plasmid pBR322 ccc-DNA as a substrate. The order of DNA strand-breaking activity in the presence of Cu2+ was (7) > (8) > or = (5) > (2) > (6) > (4) > or = (1) > or = (3). 2,5-Bis(D-arabino-tetrahydroxybutyl)-2,5-dihydropyrazine (1) and 2,5-dihydro-3,6-dimethylpyrazine (2) have already been described in terms of DNA breaking activity in a previous paper. The activity was suggested to be due to the dihydropyrazine skeleton, in addition to active oxygen radicals formed in an aqueous solution. The introduction of a methyl group to the dihydropyrazine skeleton enhanced the activities of dihydropyrazine derivatives. The possible chemical basis for DNA strand breakage by dihydropyrazine derivatives, especially in the presence of Cu2+, was discussed.

Dehydrochlorination of D-glucosamine (2-amino-2-deoxy-D-glucose) hydrochloride with an anion exchange resin made its DNA breaking activity in plasmid pBR322 much higher, especially in the presence of Cu2+. The sample of anion exchanger-treated D-glucosamine hydrochloride, i.e., HCL-free D-glucosamine sample, showed an absorption maximum at 274 nm on the UV absorption spectrum in water as seen in the case of fructosazine [2,5-bis(D-arabino-tetrahydroxybutyl)pyrazine] one of the dimers of D-glucosamine. On a positive-ion fast atom bombardment (FAB) mass spectrum, the sample showed an ion peak at m/z 323 as a base peak, corresponding to dihydrofructosazine [2,5-bis(D-arabino-tetrahydroxybutyl) dihydropyrazine], which was a precursor of fructosazine, as well as those of D-glucosamine itself (m/z 180) and fructosazine (m/z 321). The DNA strand breaking activity of HCL-free D-glucosamine sample was directly proportional to the peak intensity of m/z 323 ion, while the DNA breaking activity of fructosazine was much weaker than that of HCL-free D-glucosamine sample. 2,5-Dihydro-3,6-dimethylprazine and 2,3-dihydro-5,6-dimethylpyrazine having a dihydropyrazine skeleton the same as dihydrofructosazine showed the same extent of DNA strand breaking activity as did the HCL-free D-glucosamine sample. These results indicated that dihydrofructosazine produced during the dehydrochlorination is closely involved in the DNA breaking activity of HCL-free D-glucosamine sample.