DNA methylation is an epigenetic event involved in a variety array of processes that may be the foundation of genetic phenomena and diseases. DNA methyltransferase is a key enzyme for cytosine methylation in DNA, and can be divided into two functional families (Dnmt1 and Dnmt3) in mammals. All mammalian DNA methyltransferases are encoded by their own single gene, and consisted of catalytic and regulatory regions (except Dnmt2). Via interactions between functional domains in the regulatory or catalytic regions and other adaptors or cofactors, DNA methyltransferases can be localized at selective areas (specific DNA/nucleotide sequence) and linked to specific chromosome status (euchromatin/heterochromatin, various histone modification status). With assistance from UHRF1 and Dnmt3L or other factors in Dnmt1 and Dnmt3a/ Dnmt3b, mammalian DNA methyltransferases can be recruited, and then specifically bind to hemimethylated and unmethylated double-stranded DNA sequence to maintain and de novo setup patterns for DNA methylation. Complicated enzymatic steps catalyzed by DNA methyltransferases include methyl group transferred from cofactor Ado-Met to C5 position of the flipped-out cytosine in targeted DNA duplex. In the light of the fact that different DNA methyltransferases are divergent in both structures and functions, and use unique reprogrammed or distorted routines in development of diseases, design of new drugs targeting specific mammalian DNA methyltransferases or their adaptors in the control of key steps in either maintenance or de novo DNA methylation processes will contribute to individually treating diseases related to DNA methyltransferases.

A flow-injection biosensor system was developed for the simultaneous assay of urea and creatinine, with a single injection and one detector. The amperometric detection of urea or creatinine was based on coupled reactions of three sequentially aligned enzyme reactors, urease or creatinine deiminase, glutamate dehydrogenase and glutamate oxidase. Ammonia produced by the enzymatic hydrolysis of urea or creatinine was converted to glutamate, and the oxygen consumption due to the oxidation of glutamate by glutamate oxidase was detected with an oxygen electrode. A split and confluence of the flow stream between the injector and the glutamate dehydrogenase reactor resulted in a two-channel system. Doublepeak recording was achieved by putting a delay coil at one of the two channels. The system gave linear calibration curves over a range of 0.1--5.0 mM for both urea and creatinine. The assay procedure is simple and one run can be completed within 3 min. The system was reproducible within 5% of the relative standard deviation.

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A flow-injection analysis biosensor system was developed for the amperometric assay of creatinine based on coupled reactions of three immobilized enzymes, using an oxygen electrode as the detection device. The ammonia produced by creatinine deiminase-catalyzed hydrolysis of creatinine was further converted into L-glutamate with two sequentially aligned enzyme reactors: glutamate dehydrogenase and glutamate oxidase. Endogenous ammonia was simultaneously compensated with a double peak recording system, where the flow was split after sample injection and rejoined before the glutamate dehydrogenase reactor. The system gave linear calibration in a range of 0.1-2.0 mM for creatinine and the first peak of ammonia, and 0.1-3.0 mM for the second peak of ammonia. One run was completed within two minutes. The system can be readily applied to the assay of creatinine in urine and showed good correlation with that from the currently used Jaffe method.