•••••••"^ 



■• ♦ ♦* 



bcl «c2 «c3 ttc4 »nl «n2 bn3 



ADULT CORTEX 



ADULT NUCLEUS 



Fig. 7. 



Electrophoretic analysis of the adult cortex and nucleus y -crystalllns. Electrophoresis 

 was carried out in 0.5 M Trls-0.021 M EDTA-0.075 W boric acid, pH 8.9, at constant volt- 

 age (5.8 V/cm) for 17 hrs. The y -crystalllns tested were prepared by DEAE-fracdona- 

 tion, precipitated in ammonium sulfate and redissolved in 0.05 M Tris. (Fig. 5, J. 

 Papaconstantinou, Biochim. Biophys. Acta 107, 81, 1965; reproduced with permission of 

 Elsevier Publishing Company.) 



tiate into fiber cells. The only functional varia- 

 tion between the epithelial cells of young and 

 adult lenses is their mitotic activity, which is 

 greater during the early periods of lens growth 

 (22-25). As the lens reaches its maximum size 

 in the adult, mitotic activity decreases. It is 

 during this decrease in mitotic activity that the 

 major transformation from LDH-5 to LDH-1 

 occurs. On this basis it might be proposed 

 that this isozymic transition is associated with 

 a slowing down of mitotic activity as well as 

 other metabolic functions. An example which 

 might be considered similar to the aging of the 

 lens epithelial cells has been reported by Dawson, 

 etal., (18). They have found that more LDH-1 

 and less LDH-5 are found in samples of human 

 muscle obtained from elderly people and that the 

 highest concentration of LDH-5 subunits is found 

 in muscle from healthy adult males. 



Now 1 would like to consider our observa- 



tions on the LDH isozyme patterns during the 

 differentiation of epithelial cells to fiber cells. 

 In addition to the changes in the epithelial cells 

 alone we found that in both the calf and adult 

 lens pronounced changes occur during the dif- 

 ferentiation of the fiber cell. This is the final 

 stage of lens cell differentiation and results in 

 the transition from a replicative cell to a non- 

 replicative cell. During this stage of differen- 

 tiation LDH-1 persists. In the calf, the fiber 

 cells contain 5 detectable LDH's in which LDH-1 

 is predominant. In the adult, the fiber cells 

 contain essentially just LDH-1 although small 

 amounts of LDH-2 are detectable. On the basis 

 that the synthesis of subunits of LDH's-1 and 5 

 are genetically regulated, the complete loss of 

 LDH's- 3, 4 and 5 in the adult lens fiber cell and 

 the intermediate trends toward this loss in the 

 calf cells might be attributed to the suppression 

 of LDH- 5 subunit synthesis which would decrease 



54 



