Cystine requirements

The conversion of cysteine to cystine requires additional comment. The —SH group of cysteine makes cysteine a thiol. One property of thiols is that they can be converted to disulfides by mild oxidizing agents. This conversion, moreover, can be reversed by mild reducing agents ... 

Pea.nuts, The proteins of peanuts are low in lysine, threonine, cystine plus methionine, and tryptophan when compared to the amino acid requirements for children but meet the requirements for adults (see Table 3). Peanut flour can be used to increase the nutritive value of cereals such as cornmeal but further improvement is noted by the addition of lysine (71). The trypsin inhibitor content of raw peanuts is about one-fifth that of raw soybeans, but this concentration is sufficient to cause hypertrophy (enlargement) of the pancreas in rats. The inhibitors of peanuts are largely inactivated by moist heat treatment (48). As for cottonseed, peanuts are prone to contamination by aflatoxin. FDA regulations limit aflatoxin levels of peanuts and meals to 100 ppb for breeding beef catde, breeding swine, or poultry 200 ppb for finishing swine 300 ppb for finishing beef catde 20 ppb for immature animals and dairy animals and 20 ppb for humans. 

In terms of amino acids bacterial protein is similar to fish protein. The yeast s protein is almost identical to soya protein fungal protein is lower than yeast protein. In addition, SCP is deficient in amino acids with a sulphur bridge, such as cystine, cysteine and methionine. SCP as a food may require supplements of cysteine and methionine whereas they have high levels of lysine vitamins and other amino acids. The vitamins of microorganisms are primarily of the B type. Vitamin B12 occurs mostly hi bacteria, whereas algae are usually rich in vitamin A. The most common vitamins in SCP are thiamine, riboflavin, niacin, pyridoxine, pantothenic acid, choline, folic acid, inositol, biotin, B12 and P-aminobenzoic acid. Table 14.4 shows the essential amino acid analysis of SCP compared with several sources of protein. 

The sulphite aftertreatment is particularly important with permonosulphuric acid treatment. Evidence for the underlying mechanism is available from analysis of sulphur oxidation products formed in the various processes (Table 10.34). It is evident from these results that the concentration of RSS()5 anionic groups necessary to change the hydration of the fibre surface is achieved by the reaction of bisulphite with cystine monoxide residues to give the required cysteine-S-sulphonate groups [311]. 

It is an aerobic, gram-negative, nonmotile, nonsporing, coccobacillus that requires cystine for growth. The organism can remain viable for weeks in soil, water, carcasses, and hides. It is resistant for months at temperatures of freezing or below. The natural reservoir is rodents, particularly rabbits and hares. This is a biosafety level 2 agent. 

A comparative study was made of the RP-HPLC analysis of free amino acids in physiological concentrations in biological fluids, with pre-column derivatization by one of the four major reagents o-phthalaldehyde (73) in the presence of 2-mercaptoethanol, 9-fluorenylmethyl chloroformate (90), dansyl chloride (92) and phenyl isothiocyanate (97, R = Ph) (these reagents are discussed separately below). Duration of the analysis was 13-40 min. Sensitivity with the latter reagent was inferior to the other three however, its use is convenient in clinical analysis, where sample availability is rarely a problem. The derivatives of 73 were unstable and required automatized derivatization lines. Only 92 allowed reliable quantation of cystine. All four HPLC methods compared favorably with the conventional ion-exchange amino acid analysis188. 

By the 1930s many workers had shown that nutritionally inadequate proteins, such as zein from maize, could be effective as a source of nitrogen if supplemented by additional amino acids (for zein, tryptophan). Even if it contained all the essential amino acids, the amount of protein in the diet influenced the results. Osbome and Mendel found that if the diet contained 18% by weight casein, which is low in cystine, young rats grew, but if the amount of protein was diminished, added cystine was required to offset the relative deficiency of this amino acid. Later, after methionine had been discovered, it was shown to replace the need for cystine. 

For the synthesis of double-stranded symmetrical and unsymmetrical monocystine peptides the formation of an intermolecular disulfide bridge is required. For homodimerization of cysteine peptides all the methods discussed in Section 6.1.1 can be applied taking into account the reactivity of the different oxidative agents toward sensitive amino acid residues present in the peptide sequences. Synthetic approaches based on the direct use of suitable cystine derivatives can be envisaged, at least for small-size peptides since disproportionation would in all cases retain the homodimeric structure 241... 

For the synthesis of heterodimeric cystine peptides where two different peptide chains are cross-linked by a disulfide bridge random co-oxidation of the two chains besides producing the heterodimer leads in the optimal case to the additional two homodimers in statistical distribution. Therefore, chemical control of the disulfide bridging via site-directed disulfide formation techniques is required since a thermodynamic control for generation of heterodimers is extremely difficult to achieve (see Section 6.1.5). 

Peptide bonds are cleaved in a nonselective, but not in a completely random manner. Based on anchimeric side-chain assistance, steric factors, and bond strains, acid-labile peptide bonds are predicted to include sites containing Asp, Glu, Ser, Thr, Asn, Gin, Gly, and ProJ22l The disulfide topologies of circulin B and cyclopsychotride, backbone-cyclized peptides with three disulfide bonds, were determined by partial hydrolysis for 5 hours.[22 Occasionally, the bond between adjacent half-cystine residues is cleaved due to the nonselective nature of the mechanism of partial acid hydrolysis.[21] By this procedure, in all cases, a complex mixture of peptide fragments is produced which requires careful chromatographic separation by RP-HPLC for subsequent analysis by mass spectrometry (see Section 6.1.6.2.7). 

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