The degradation of L-histidine and trans- and cis-urocanic acid by bacteria from skin and the role of bacterial cis-urocanic acid isomerase

UV-B radiation suppresses cell-mediated immunity. Histidine forms hans-uroc anic acid (trans-UCA) enzymatically in the stratum corneum. Photoisomerizat ion of trans-UCA to cis-urocanic acid (cis-UCA) has been proposed for the i nitiation of an immunosuppressive process. Many microorganisms described in the literature metabolize histidine and/or trans-UCA. Our enrichment cultu res of soil and sewage contain organisms that can degrade cis-UCA. We have tested microorganisms for degradation of cis-UCA, trans-UCA, or L-histidine when they are incorporated at 0.2% in nutrient broth. Six out of 10 select ed genera isolated by our clinical microbiology laboratory degrade one or m ore of the imidazole substrates. We have cultured over 60 aerobic isolates from human skin. Of these, 33 degrade one or more of the three imidazole su bstrates and 12 degrade cis-UCA. Isolates from BALB/c mice are also active on cis-UCA. We have identified a cis-UCA-degrading bacterium as Micrococcus luteus. Four ATCC strains of M. luteus have been tested and three are acti ve on histidine or trans-UCA; two are active on cis-UCA. Micrococci that de grade cis-UCA contain anew enzyme, cis-UCA isomerase, which converts the su bstrate to the trans-isomer. This enzyme provides access to the classical L -histidine degradation pathway. We hypothesize that an epidermal microflora that degrades L-histidine, trans-UCA, or cis-UCA influences the concentrat ion of urocanic acids on the skin and, thus, affects immune suppression. (C ) 1999 Elsevier Science S.A. All rights reserved.