DL-Isovaline

C,H,)fi-COOH, CsHsCOOH, NOfi H.COOH Tartrate buffer Tartaric acid Salicylic acid H NCHfiOOH, dl-Isovaline, Histidine, ff-Alanine EDTA... 

The purified enzyme displays its highest activity at 50°C. Furthermore, the enzyme is fairly stable because more than 90% of the activity is retained after a preincubation for 30 min up to 55°C. The L-aminoamidase is active between pH values of 6.5-11.0, with a broad pH optimum at pH values of 8.0-9.5 (Fig. 9). Determination of the substrate specificity of the purified enzyme showed that this amide hydrolase was active toward a rather broad range of bodi a-H- and a-alkyl-substituted amino acid amides (Table 8). Within these two substrate classes, highest activity is observed for file cyclic amino acid amide DL-proline amide and the a-alkyl-substituted amino acid amides DL-isovaline amide and DL-a-allylalanine amide. Furthermore, this enzyme displays modest activity toward a number of simple amides (e.g., acetamide and propionamide). The enzyme appears to be inactive with the a-hydroxy acid amide DL-mandelic acid amide and with the dipeptide L-Phe-L-Leu. These results clearly show that this purified amide hydrolase belongs to the group of aminoamidases. 

DL-a-Methylleucine amide DL-Isovaline amide DL-a-AUylalanine amide + + -I- L-Phe-L-Leu ... 

UV radiation hypothetical, but so is the transport of molecules from outer space to Earth. Recent analyses of the Murchison meteorite by two scientists from the University of Arizona, Tucson (Cronin and Pizzarello, 1997 Cronin, 1998) have shown it to contain the four stereoisomeric amino acids DL-a-methylisoleucine and DL-a-methylalloisoleucine. In both cases, the L-enantiomer is present in a clear excess (7.0 and 9.1%). Similar results were obtained for two other a-methyl amino acids, isovaline and a-methylvaline. Contamination by terrestrial proteins can be ruled out, since these amino acids are either not found in nature or are present in only very small amounts. Since the carbonaceous chondrites are thought to have been formed around 4.5 billion years ago (see Sect. 3.3.2), the amino acids referred to above must have been subject to one or more asymmetric effects prior to biogenesis. 

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