Zinc, Growth, & Respiration in Euglena

Abstract

Zinc is essential for normal growth in plants, animals, and microorganisms (cf. reviews 1, 2,17). What is the biochemical basis for this essentiality? One answer to this question can be made by simply citing the burgeoning number of zinc metalloenzymes (12). fact that any one enzyme contains zinc as an essential component does not necessarily mean that the activity of this enzyme decreases in zinc deficiency. Much less does it mean that the activity of this enzyme limits the growth of the zincdeficient organism. means for obtaining a more valid answer to the question of the biochemical basis for essentiality were stated by Professor Pirson (9): The first criterion includes the requirement that all biochemical and especially enzymological evidence must be critically tested for validity in vivo; i.e., the living plant itself must always be asked. In seeking the biochemical basis for the essentiality of zinc our strategy is to examine broad areas of metabolism for a lesion occurring in zinc deficiency. Thc lesion, or failure in some process, must be of sufficient degree to account for the corresponding failure of growth. Upon discovering such a lesion, we should then try to identify the enzyme or other reactions responsible. Since the pyridine nucleotide (PN) -dependent dehydrogenases are among the zinc metalloenzymes previously discovered, we have first examined respiration for a possible growth-limiting lesion in zinc deficiency. test plant used throughout is Euglena gracilis, an organism particularly suitable for physiological and biochemical studies involving zinc. Part of these results were previously communicated to the American Society of Plant Physiologists (6).