The Effect of Environment on Cystine Disruption by Ultraviolet Light*

Abstract

When cystine is irradiated at pH 1 by 254‐nm u.v. the following yields are observed: 4 cystines → 5.2 cysteines + 2.8NH3. Thus, SH production accounts for only 0.65 of the cystine destruction; further C‐S breakage to give alanine or serine is not efficient. The yields for cystine and glutathione destruction are essentially the same at pH 1. However the presence of the glutamic and glycine residues stabilize the cystine in glutathione so that NH3 is not lost until the peptide bonds are hydrolyzed. Increasing the pH from 1 to 8.6 increases the yield of cystine destruction in glutathione by 50 per cent. The yield of cystine destruction is greater in both compounds when O2 is present during irradiation (e. g. the cysteic acid yield in glutathione is increased by 50 times). The overall production of SH varies by a factor of 2 in the four proteins‐insulin, RNase, trypsin and lysozyme. The present data further support the earlier observation that radiation damage is quite non‐random in RNase: at least two and perhaps three of the four constituent cystines must be disrupted before activity is lost: i.e. the most radiosensitive cystines are not critical for enzymic activity. Similarly, in both trypsin and lysozyme the integrity of the most radiosensitive cystines also does not appear to be critical for the retention of enzymic potential. In insulin, however, all three cystines appear to be crucial for activity and to have approximately equal radiosensitivities. These differences in sensitivity of cystines in different proteins must depend specifically upon energy transfer and/or chemical interactions between the chromophoric groups. If yields are calculated on the basis of those quanta absorbed only in the cystines, values about 5 to 8 times greater than those in the model compounds cystine and oxixized glutathione are obtained. The yields of cystine destruction are much higher in those protiens which contain trypotophan.

High probability of disrupting a disulphide bridge mediated by an endogenous excited tryptophan residue

“…The photosensitivity of protein-bound cystine residues has been shown to depend on their microenvironment (Augenstein and Ghiron 1961;Dose and Rajewsky 1962;Augenstein and Riley 1964;Dose 1964Dose , 1967Fiore and Dose 1965;Grist et al 1965;Risi et al 1967;Koudelka and Augenstein 1968). This correlates with the observation by Petersen et al (1999) that cystine residues (cysteines involved in disulphide bridges) have a clear preference for aromatic residues as spatial neighbors.…”

“…This correlates with the observation by Petersen et al (1999) that cystine residues (cysteines involved in disulphide bridges) have a clear preference for aromatic residues as spatial neighbors. There is evidence that quanta primarily absorbed in the side-chains of aromatic amino acid residues of a given protein molecule contribute to the destruction of the cystine residues (Dose 1967;Risi et al 1967;Koudelka and Augenstein 1968). It has been reported that the UV illumination of Trp and Tyr residues gives rise to electron or H-atom ejection from these amino acids (Vladimirov et al 1970;Feitelson 1971).…”

Photochemical Reactions in Amino Acid Residues and Inactivation of Enzymes During U.V.‐IRRADIATION. A Review

“…Tyrosine photoproduct spectra were compared with the spectra of irradiated 0acetyltyrosine in which, in contrast to the tyrosine, the hydrogen atom of the hydroxyl group is replaced by the residue of acetic acid. It was shown that the irradiated 0acetyltryosine has a luminescence spectrum similar t o that ot the tyrosine product (the maximum is near 5 16 and 490 nm, Fig. 3).…”

Reversible Hydrogen Transfer Reactions of Cysteine Thiyl Radicals in Peptides: the Conversion of Cysteine into Dehydroalanine and Alanine, and of Alanine into Dehydroalanine

“…The formation of Ala during the photo-irradiation of cystine had been recognized several decades ago, but a satisfactory mechanism had not been postulated. 26–29 …”