Orthogonal, Spectroscopic High Throughput Screening of Laccase-Catalyzed p-Cresol Oxidation

Abstract

There is considerable interest in the oxidative fate of phenols such as p-cresol as environmental pollutants and uremic toxins. We supply a menu of spectroscopic options for the high throughput screening of laccase oxidation of p-cresol through multiple modes of detection. Laccase activity was monitored kinetically at pH 4.5 by absorption changes at 250 nm, 274 nm or 297 nm, and in endpoint mode by the bathochromic shift in absorption to 326 nm in 50 mM NaOH. Laccase oxidation of p-cresol was also detected by product fluorescence at 425 nm after excitation at 262 nm or 322 nm in 50 mM NaOH. We optimized the kinetic parameters for p-cresol oxidation (pH optimum 4.5-5.1; 37 degrees C; Km = 2.2 mM) resulting in laccase limits of detection and quantitation of 25 pg/microL and 75 pg/microL, respectively (approximately 360 pM; 25 ppb). The sensitivity for p-cresol was similar to previously reported values. The small (approximately 20%) decrease in signal strength after six cycles of excitation over a 3 h period was attributed to photobleaching or photodegradation of the emitter and not due to fluorescence decay (photoinstability). Halide inhibition was characteristic of laccases (IC(50) = 25 mM NaCl). A unique advantage of our assay is that laccase catalysis could be interrogated using multi-mode absorption or fluorescence under acidic or basic conditions, in real time or endpoint modes. Orthogonal interrogation facilitates ratiometric analysis enabling high specificity while minimizing interferences during compound library screening. The phenolic alcohol p-cresol may be a model for monolignol oxidation. Our studies might find applications in biofuels, to triage dialysis patients, or for the environmental bioremediation of phenols.