Mutants ornithine

Ornithine transcarbamylase deficiency. This is the most common of the urea cycle defects. Presentation is variable, ranging from a fulminant, fatal disorder of neonates to a schizophrenic-like illness in an otherwise healthy adult. Males characteristically fare more poorly than do females with this X-linked disorder because of random inactivation (lyonization) of the X chromosome. If inactivation affects primarily the X chromosome bearing the mutant OTC gene, then a more favorable outcome can be anticipated. Conversely, the unfavorably lyonized female has a more active disease. 

Recently, we have modeled9 intrinsic carbon kinetic isotope effects on the ornithine decarboxylase-catalyzed decarboxylations. Decarboxylations occur from the pyridoxal 5 -phosphate (PLP) - substrate complexes. These reactions provide a good model case since a number of 13C kinetic isotope effects for the wild-type enzyme and its mutants, as well as for physiological and slow substrates, have been reported.10 Using AM1/CHARMM/MD calculations on nearly 18000-atom models... 

At least two mutant alleles of ornithine delta-aminotransferase cause gyrate atrophy of the choroid and retina in Finns. Proc Natl Acad Sci USA 86 197-201. 

Methods employing mutants (L-lysine, L-threonine, L-arginine, L-citrulline, L-ornithine, L-homoserine, L-trypophan, L-phenylalanine, L-tyrosine, L-histi-dine, etc.)... 

Many kinds of amino acids (eg, L-lysine, l-ornithine, L-phenylalanine, L-threonine, L-tyrosine, L-valine) are accumulated by auxotrophic mutant strains (which are altered to require some growth factors such as vitamins and amino acids) (Table 6, Primary mutation) (22). In these mutants, the formation of regulatory effector(s) on the amino acid biosynthesis is genetically blocked and the concentration of the effector(s) is kept low enough to release the regulation and induce the overproduction of the corresponding amino acid and its accumulation outside the cells (22). 

Shih V E. (1973) Laboratory Techniques for the Detection of Hereditary Metabolic Disorders CRC, Boca Raton, Florida Sipila I., Simell O., and O Donnell J. ] (1981) Gyrate atrophy of the choroid and retina with hyperornithinemia Characterization of mutant liver L-ornithine 2-oxoacid aminotransferase kinetics / Chn. Invest. 67, 1805-1807... 

Mutant group Growth with Ornithine Growth with Citrulline Growth with Arginine Site of block... 

The detailed and complete reaction sequence was finally elucidated by the use of further mutants (blocked at various stages between glutamate and ornithine), by the additional use of isotopic methods, and by the characterization of the individual enzymes. 

Baumberg S, Mountain A. Bodllus subtilis 168 mutants resistant to arginine hydioxamate in the presence of ornithine or citrulline. J Gen Miccobiol 1984 130 1247-1252. 

A third allelic variety is one which produces an arginine requirement for growth. Mutants of this type have been found in E. coli [77] and Salmonella [76a]. In the latter case, the auxotrophic phenotype is expressed only at low temperatures. The mutant enzyme has one-sixth the activity of the wild-type enzyme and is remarkedly altered in its allosteric properties. At 20°, as compared with 37°, the inhibition by UMP is increased, and activation by ornithine is decreased. The result is a cold-sensitive arginine auxotroph. A fourth form is one in which growth is inhibited by arginine and restored by uracil [76a,78]. Although the enzyme activity of this mutant type is considerably reduced, the exact nature of the regulatory alteration is yet to be determined. Table III summarizes the different allelic forms that have been obtained by mutational events in the pyrA locus. 

Figure 3 also illustrates acetylornithinase formation in an argR mutant of strain W, followed as described above (plot 3). In this case, comparable results are obtained with or without added arginine. The ratio of the slope of plot 3 to that of plot 2 (repressibility ratio) is of the order of magnitude of 10. This ratio is low compared to that for ornithine transcarbamylase, which has a value of several hundred. 

The glutamate-ornithine relationship has been confirmed by the use of isotopes and by the discovery of a PenidUium mutant responding to either substance. 

Study of the pathway from ornithine to citrulline, using mutants of Neurospora, has shown that there are two stages. The first requires ATP CO2 and NH3 are fixed to form an as yet unidentified compound, which is then transformed into citrulline. 

---