Methionine methyl ester hydrolysis

Methionine-enriched protein was produced also from an enzymatically prehydrolyzed milk protein using an enzymatic peptide modification method with a-chymotrypsin as catalyst. Amino acid incorporation leading to methionine enrichment of the product proceeded via formation of covalent bonds. The concentration of the substrate was 25% (w/v). Methionine was added to the reaction mixture in the form of methionine methyl ester hydrochloride. An ester/substrate ratio of 1 5 was used in the enzymatic peptide modification reaction. The methionine content of the product was twice as high as that of the substrate. The slight change in the degree of hydrolysis revealed that part of the amino acids were bound to the peptide chains and that transpeptidation was the main force during this enzyme-catalyzed reaction. The newly incorporated Met was located in C- and N-termi-nals in a ratio of 3 1 [82],... 

Functional properties of some enzymatically modified and EPM-treated products of milk proteins [136] were determined as follows. An enzymatically prehydrolyzed commercial milk protein concentrate (SR) without further hydrolysis, and casein hydrolyzed by alcalase, a-chymotrypsin, and papain, respectively, were used as substrates in the EPM reaction. The concentration of the hydrolysates was 20% w/ v in the EPM reactions. A methionine methyl ester hydrochloride/ substrate ratio of 1 5 was used for incorporating this amino acid. After incubation, the products with methionine incorporation were simultaneously dialyzed for 2 days through a cellophane membrane against distilled water. The nondialyzable fractions and the EPM products without amino acid enrichment were freeze-dried. Covalent methionine incorporation in the EPM products with amino acid enrichment was verified by exopeptidase hydrolysis of the protein chains. The functional properties of the different EPM products are summarized in Table 1. An important functional property of proteins and/or peptide mixtures is their emulsifying behavior. This is highly influenced by the molecular structure, the position and ratio of hydrophobic-hydrophilic amino acids. Emulsion activity was found to be low (34.0) for casein, and the values determined for enzyme hydrolyzed and modified products were in general even lower. The papain hydrolysate, sample H3, showed here a different behavior as well this was the one of the sample series that had the highest EAI value (43.0). The emulsion stability of the enzymatically modified products displayed tendencies quite opposite to the values of emul-... 

More detailed studies of reactions of this type have been reported.96,97 Nickel(II) complexes of histidine and tryptophan provide stereoselectivity in the hydrolysis of histidine methyl ester, but stereoselectivity is not observed with nickel(Il) complexes of aspartic acid or methionine. Only tridentate ligands with a minimum steric bulk appear to be capable of exhibiting stereoselectivity in reactions of this type. 

Yamashita et al. (65) incorporated L-methionine into a soybean protein hydrolysate by means of the plastein reaction with papain. A 10 1 mixture of a peptic hydrolysate of soybean protein isolate and L-methionine ethyl ester was incubated in the presence of papain, the conditions being similar to those mentioned above. The methionine content of the plastein was 7.22 wt %, nearly seven times the original methionine content of the soybean protein isolate. To determine the location of the incorporated methionine residues, the plastein was treated with carboxypeptidase A. Methionine was liberated much faster than any other amino acid. A second portion of the plastein was methylated and then treated with lithium borohydride to reduce the COOH to CH2OH. Hydrolysis of the chemically treated plastein with 6N HC1 gave aminols in satisfactory yields. Subsequently, the aminols were converted to their DNP-derivatives, which were separated by thin layer chromatography. These experiments, together with some others, showed that 84.9% (molar basis) of the C-terminals of the plastein molecules were occupied with methionine, whereas only 14.4% of the N-terminals contained methionine. 

The origin of the additional carbon atoms of insect juvenile hormone (49) is not yet settled. Methionine seemed to be incorporated only into the ester methyl group. - Mevalonate was incorporated into farnesol but not into juvenile hormone. A slight incorporation of [2- " C]acetate into the chain was noted but not of [l- C]acetate. Metabolism of the ester includes hydrolysis to the corresponding acid and possibly formation also of the corresponding diol from the epoxide group. 

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