There are many examples of enzymes that share substrates of cofactons in a cyclic manner. Techniques have been developed that use cyclic enzyme systems to assay quantitatively small amounts of biochemical substances. Physiological importance of cyclic enzyme systems is well known, but only few studies on the control property of these systems have been published. We investigated the dynamic characteristics of cyclic enzyme systems using computer simulations. They display catastrophic behavior in response to specific changes in external perturbations. Based on the results, we describe a role of "biochemical switching circuit" of cyclic enzyme systems in metabolic pathways.
Most of the investigations of ultraviolet(UV)-radiation effects on organisms have been made with 254nm from a germicidal lamp. The specific DNA lesion, pyrimidine dimers, caused by UV irradiation have been shown to be toxic, mutagenic and carcinogenic. In contrast, knowledge of the biological effects of far UV(UV-C) radiation may not be relevant to the carcinogenic, mutagenic and lethal effects of UV in sunlight. Recently, since it has been indicated that the DNA lesions caused by mid-UV(280-320nm, UV-B) and nearUV(320-400nm, UV-A) is harmful to a wide variety of organisms, the studies of mid-UV and near-UV effects are considered to be important. The possible DNA damages caused by mid-UV and near-UV are summarized in the present review. In mid-UV range, both the pyrimidine dimers and other photoproducts such as 5-hydroxy-methyl cytosine may be responsible for the observed biological effects and they are repairable by excision repair systems or T4 endonuclease. Several DNA damages of near-UV are indicated to be DNA strand breaks dependent on pol+ and nur+ repair systems, DNA-protein cross-links and a small amount of pyrimidine dimers which are responsible to repair deficint mutants including human patients. In addition, other kinds of photodamages in tRNA carrying cytidine 13 and thiouridine 8 link than DNA damage are indicated to also interact to repair processes.
Recent advanced technologies of molecular cloning and protein chemistry revealed the entire primary structure of various cytochrome P-450 species so that the multiplicity of P-450 was evidently clarified. The domains responsible for heme binding and subcellular localization were posturated on the basis of the comparison of their primary structures. The rat microsomal P-450MC cDNA was efficiently expressed in the yeast Sacchromyces cerevisiae under the control of alcohol dehydrogenase I promoter and terminator. The P-450MC protein synthesized in yeast was localized in microsomes, contained heme in the molecule and interacted with an yeast electron-transport chain to exhibit monooxygenase activity. The yeast host-vector system combined with the site-directed mutagenesis and chimeric gene construction can be useful for overproduction and characterization of membrane-bound P-450s.