Calf 

 epith cortex 



ORGIN 



Adult 

 epith cortex 



ORGIN 



2 

 3 



4 



— >■ 

 5 



Fig. 9. 



A diagramatic presentation of the LDH isozyme patterns 

 for calf and adult lens epithelial cells and fiber cells. 

 (Fig. 7, J. Papaconstantinou, Science, in press; copyright 

 1966 by the American Association for the Advancement 

 of Science.) 



adult heart tissue of mouse and chicken, while 

 LDH-5 is the predominant form in tissues that 

 can function under conditions of oxygen debt, 

 such as adult skeletal muscle. Furthermore, it 

 has been shown that LDH-1 is more sensitive to 

 inhibition by high pyruvate concentrations than 

 LDH-5 (14, 15, 17). On the basis of these ob- 

 servations it has been postulated that in highly 

 oxidative tissues such as the heart, the level of 

 lactic acid is regulated, i.e., kept at a low level, 

 because of the sensitivity of LDH-1 to pyruvate. 

 This hypothesis is further borne out by the fact 

 that skeletal muscle, which is capable of tolerat- 



ing a greater variation in oxygen tension than 

 heart muscle, contains more active LDH-5, the 

 isozyme which shows less sensitivity to sub- 

 strate inhibition. 



Let us now consider the metabolic proper- 

 ties of the lens cells. Wanko and Gavin (25, 26) 

 reported that the epithelial cells have relatively 

 more mitochondria than the fiber cells and that 

 the population of epithelial cell mitochondria is 

 significantly decreased after fiber cells are 

 formed. Thus, metabolically the epithelial and 

 fiber cell differs significantly in that the former 

 cell type exhibits a greater degree of aerobic, 

 oxidative metabolic pathways. Epithelial cells 

 have been shown to have higher levels of cyto- 

 chrome c, greater succinate dehydrogenase 

 activity, and more active mitochondria (27). 

 Fiber cells, on the other hand, have been shown 

 to have a greater degree of aerobic glycolysis 

 (28). Furthermore, it has been shown that the 

 most efficient production of ATP from ADP 

 in calf cortex fibers occurs with fructose-1, 

 6-diphosphate as substrate (29). Krebs cycle 

 enzymes are detectable in fiber cells, but their 

 activity is significantly less than that found in 

 the epithelial cells. All of these observations 

 indicate that a major metabolic difference be- 

 tween epithelial and fiber cells is in their 

 respiratory and glycolytic activity. 



Taking the metabolic properties of lens 

 cells into account it would appear from the 

 work on heart and skeletal muscle LDH that 

 the lens fiber cells should retain LDH-5. Our 

 data have shown the opposite, i.e., that the 

 fiber cells retain LDH-1. In addition, even 

 though LDH-1 is retained, high lactic acid 

 levels are maintained by these cells. 



Several factors such as oxygen tension, 

 intracellular pools of metabolic intermediates 

 and cofactors, and predominating pathways of 

 carbohydrate metabolism have been postulated 

 to play an important role in the type of LDH 

 isozymes retained by a specific tissue (30, 31). 

 Recent work on the LDH isozymes in cultured 

 chick heart muscle cells has shown that after 

 6 days in culture LDH-5 is the predominant 

 form (30). Prior to being placed in culture these 

 cells have predominantly LDH-1. In fact, chick 

 heart cells have been shown to retain LDH-1 

 throughout embryonic and post-embryonic life. 

 Thus, under tissue culture conditions a new 

 phase of LDH isozyme distribution, not previ- 

 ously experienced by these cells, is developed. 

 This predominance of LDH-5 was significantly 

 slowed down when placed under conditions of 

 high oxygen tension or when Krebs cycle inter- 

 mediates are added to the culture medium. 



56 



