Cysteine oxidized, absorption spectra

The second modified flavin of natural origin to be discovered was 8a-S-cysteinyl-FAD, the coenzyme of monoamino oxidase from liver and kidney outer mitochondrial membranes. Taking their departure from investigations of Yasunobu (8J) and Hellerman (SO), which indicated the presence of covalently bound flavin in preparations of this enzyme, Singer and his group (85, 185) isolated the flavinyl peptide by degradation of MAO with trypsin-chymotrypsin and identified cysteine as the amino acid residue bound next to the flavin moiety (184). The absorption spectrum of the flavin peptide from monoamino oxidase is readily differentiated from that of riboflavin by a hypsochromic shift of the second absorption band (360 nm, compare with 372 for riboflavin), in the neutral oxidized state (44, 184). It is similar to that of 8a-histidyl-riboflavin in the cationic state in that the band centered around 400 nm (abs. max. 375 nm, shoulder at 410 nm) is partially resolved. The fluorescence emission (4, 30) is only 10% of that of riboflavin, but oxidation with peracids raises it to 90% of riboflavin emission. 

In an article published independently at the same time, Stuehr and Ikeda-Saito89 used the purified bNOS and iNOS to reach the same conclusions. While the paper was under review, the authors mention that White and Marietta had reported earlier that the iron was a heme and was used as an oxidant. Using the same type of study, they found that the iron prophyrin and its CO derivative had the expected properties and proposed that the iron is penta-coordinated, with a cysteine thiolate as the fifth coordinate. A third publication confirmed the results when McMillan and coworkers90 used bNOS grown in human kidney cells. These workers obtained similar data for the light absorption of the enzyme and its CO spectrum. They also speculate on very similar sequences in the three types of purified enzyme that might be the porphyrin binding site. 

Transformation of the molecular structure as a result of the ozone-induced free radical oxidation of the different samples of fibrin-stabilizing factor was studied by techniques of vibrational spectroscopy. On FTIR spectrum of pFXIII there are many well-defined absorption bands in the region of valence vibration of X-H (3000-2400/cm) and in low-frequency region (1300-500/cm). Both of these areas may be a good source of structural information in this case. Particularly the bands of S-H valence vibration of cysteine residues in protein appear in the region of 2520-2600/cm,... 

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