FRANK H. JOHNSON 269 



ness of a culture, under the most favorable conditions known, varies 

 widely. Psychrophilic, marine species, such as Photohacteriiim phos- 

 phoreum, exhibit perhaps the most brilHant luminescence. The lumi- 

 nescence of certain mesophihc, marine species, such as Photobac- 

 terium splendidum or its very close relative, A. harveyi (which should 

 perhaps be considered a subspecies and designated "Photohacteriiim 

 splendidum harveyi") is greatly enhanced by the addition of fish ex- 

 tract to the medium. The significance of the fish extract is not yet 

 known, but the occurrence in fish of vitamin A in the form of the 

 palmitate suggests a possible interpretation, viz., that of providing a 

 source of palmitic aldehyde, very small concentrations of which have 

 recently been found to increase by more than 100-fold the intensity 

 of luminescence in certain extracts of luminous bacteria (Strehler, 

 1953; Cormier and Strehler, 1953). 



Turning now to the problem of inhibition and activation of lumi- 

 nescence in "mature" cells, after they have grown and developed their 

 light-emitting system, the following list includes some of the chief 

 principles that are more or less of general interest. Since the evidence 

 and theory are discussed at length elsewhere (Johnson, Eyring, and 

 Pohssar, 1954), they need not be considered in detail here. 



Reversible Denaturation of Intracellular Enzymes 



Intracellular luminescence, Hke essentially all biological processes, 

 exhibits an optimum temperature, or temperature for maximum, over- 

 all reaction rate. The actual temperature varies somewhat among dif- 

 ferent species, and within a single species it may be reversibly raised 

 or lowered by physical and chemical changes in the environment of 

 the cells. Under given conditions, a reaction of fundamental impor- 

 tance in determining the optimum temperature, and in part the tem- 

 perature activity curve, is the reversible thermal denaturation of an 

 essential enzyme. Qualitative evidence for this reaction resides in the 

 ready reversibilit>% by cooling, of the diminution in luminescence in- 

 tensity during momentary exposures to temperatures well above the 

 normal optimum. Analyses of quantitative data, relating the amount 

 of reversible diminution in intensity to various temperatures above 

 the optimum, indicate that a single reaction, characterized by the 

 high heat and entropy typical of protein denaturation, is primarily 



