The Action of Ultraviolet Radiation on Yeast Catalase

Abstract

The effect of prolonged UV irradiation (mostly 2537 A) on the catalase activity of an aqueous yeast suspension was divisible into 4 periods. First, the period during which the cells lost their ability to form colonies, but during which no change in catalase activity was noted. Second, the period during which a considerable rise in catalase activity (Euler effect) occurred. The Euler effect was accompanied by enzyme alteration as shown by the simultaneous decrease in the activation energy of the enzyme-substrate system. However, during the initial phase of this period, as the catalase activity of the suspension began to increase, the activation energy rose to a transient level higher even than that characterizing the unaltered enzyme. Heat accelerated the rate of alteration when applied either during or after the irradiation; the activation energy for the over-all alteration reaction was 24 kcal., a value close to that recorded previously for alteration induced by chemical agents. Nevertheless, the rate-limiting step appeared to be different in the two cases. A model of these events was presented in which the primary photochemical action was on the site at which catalase is located within the cell. Third, a rather long period during which irradiation led to no diminution in the catalase activity of the maximally active suspension. This protection effect was duplicated in intro by a model crystalline catalase-KNA system, or by adding either ribonuclease digestion products of RNA or adenine to a catalase solution prior to irradiation. Evidence was adduced that the protection effect was not a simple screening, but involved some sort of interaction between the enzyme and the nitrogenous components of RNA, an interaction which must likewise occur within the cell. Alteration induced by CHCl3 did not eliminate the protection effect, but that by butanol did. The onset of photoinactivation was due to modification of protein structure, not of RNA. Fourth, the period of photoinactivation of the intracellular enzyme, which was quite similar to that of the crystalline enzyme in vitro.

Comparison of the Effects of Ultraviolet Radiation on the Intracellular Catalase of Aerobic and Anaerobic Yeast

“…5 kcal but required about 40 min for this change to be produced. The transient rise in activation energy previously reported (10) was not observed in these experiments. As in our earlier studies (In), the activation energy of the aerobic enzyme-substrate system dropped as the activity of the suspension increased.…”

The enzymic composition of nuclei isolated from radio-sensitive and non-sensitive tissues with special reference to catalase activity

“…Evidence pertaining to this latter possibility is seen in the work of Brown (1952), who found a more active catalase component which was associated with structural elements of the cell and was released by vigorous homogenization. Kaplan & Paik (1957) concluded from their studies of the action of u.v. light upon yeast-cell catalase that the enzyme was associated with ribonucleoprotein, and that radiation led to the dissociation of this complex.…”