PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Preliminary characterization of polyphenol oxidase from Larrea tridentata leaves. David Becker^*,1, John Golbeck², ¹ Dept. Biology, Claremont, CA, USA² Dept. Biochemistry and Molecular Biology, USA

ABSTRACT- Polyphenol oxidase is an enigmatic enzyme that resides in the lumen of chloroplast thylakoids and is virtually ubiquitous in green plants (Sherman et al., 1995). Polyphenol oxidase (PPO) has two enzymatic activities: o-hydroxylation of monophenols (or the monophenolase activity: EC 1.14.18.1) and oxidation of o-diphenols to o-diquinones (catecholase activity: EC 1.10.3.2). Mayer (1987) referred to its catecholase activity as an enzyme in search of a function. We have begun an examination of PPO in Larrea tridentata, a perennial shrub of the desert U.S. southwest. L. tridentata is very thermotolerant, maintaining its leaves and continuing photosynthesis throughout the summer season. L. tridentata thylakoids demonstrate very high levels of PPO activity, well over 1500 micromoles oxygen/mg Chl per hour. While most leaf PPO activity requires activation by linolenic acid, detergent, or heat, this PPO is fully active in isolated thylakoids. It exhibits an acidic pH optimum of 5.0 to 6.0, depending on substrate, which is consistent with PPO having an important role during light conditions. L. tridentata PPO oxidizes a wide range of substrates, including the potentially physiological substrates chlorogenic acid and nordihydroguaiaretic acid (NDGA). PPO in L. tridentata thylakoids is inhibited by typical PPO inhibitors, including diethyldithiocarbamate (DETC), ascorbate, L-cysteine, tropolone, metabisulfite and cinnamic acid. It is most sensitive to DETC and least sensitive to the chloroplast component, ascorbate. PPO activity in L. tridentata thylakoids remains high over a wide temperature range, from 15 degrees C to at least 40 degrees C. Therefore we suggest that PPO may play some role in the thermotolerance exhibited by Larrea tridentata.

KEY WORDS: thermotolerance, polyphenol oxidase