High Quality Content by WIKIPEDIA articles! p-Hydroxyphenylpyruvate dioxygenase (HPPD) is an enzyme found in both plants and animals which catalyzes the catabolism of the amino acids phenylalanine and tyrosine. Inhibition of this enzyme has profound, but quite different, effects on both plants and animals. In plants inhibition of HPPD affects the formation of homogentisic acid, which is a key precursor for the biosynthesis of both tocopherols (vitamin E) and plastoquinone, a critical co-factor in the formation of carotenoids, which protect chlorophyll in plants from being destroyed by sunlight. If this happens,the plants turn white without deformation due to complete loss of chlorophyll, and the plant dies. Hence, the biological activity is highly selective to plants . Inhibition of HPPD in animals blocks the formation of metabolites such as succinyl acetoacetate in the liver. For some people, the presence of these metabolites can have very toxic effects including death.