Free radical scavenging potential of L-proline: evidence from in vitro assays

An assessment of the potential of proline to scavenge free radicals was made in a couple of in vitro assay systems, namely graft co-polymerization and autooxidation of pyrogallol. Both these assays are essentially dependent upon free radical mechanisms. Graft co-polymerization involved a ceric (Ce(4+)) ion- or gamma-radiation-induced grafting of methyl acrylate (MA) onto a cellulose backbone. The degree of grafting, measured gravimetrically, was taken as a measure of free radical generation. The gamma-radiation-dependent grafting was far greater than that due to Ce(4+) ions. Inclusion of proline in the assay, irrespective of the initiator used, led to suppression of grafting in a concentration-dependent manner indicating the ability of proline to scavenge free radicals. The gamma-radiation-dependent grafting was also suppressed by hydroquinone and glutathione but not by ascorbate, glycine and spermine. In contrast to graft co-polymerization, proline did not inhibit the autooxidation of pyrogallol, a reaction involving superoxide radical generation. A subset of data constitutes an evidence for the ability of proline to scavenge free radicals in vitro. It is implied by extension that free proline, known to accumulate in plant tissues during abiotic stresses, would contribute to scavenging of surplus free radicals produced under a variety of abiotic stresses.