Nucleic Acid Metabolism in Regenerating Rat Liver VI. Soluble Enzymes Which Convert Thymidine to Thymidine Phosphates and DNA*t

T h e presence of a m u l t i e n z y m e s y s t e m wh ich i nco rpo ra t e s t h y m i d i n e in to D N A in t he soluble f r a c t i o n of r e g e n e r a t i n g r a t l ive r h o m o g e n a t e s (5, 6) ra ises t h e ques t i on of t h e b io logica l sign i f icance of th i s e n z y m e sys t em. T h e r a t e of D N A s y n t h e s i s in vivo as a f u n c t i o n of t i m e a f t e r p a r t i a l h e p a t e e t o m y h a s been s tud ied in severa l l a b o r a tor ies , w i t h a v a r i e t y of i sotopic s u b s t r a t e s (1, 11-13, a n d re fe rences c i ted) . T o e x a m i n e t h e rel a t i o n b e t w e e n D N A s y n t h e s i s in vivo a n d a c t i v i t y of t h e soluble e n z y m e s y s t e m , a t echn ic for s t u d y ing these p h e n o m e n a s i m u l t a n e o u s l y in t h e s ame a n i m a l was used, fo l lowing t h e p l a n p r e v i o u s l y used to co r re la t e D N A s y n t h e s i s in vivo a n d in t i s sue slices (13). T h e resu l t s h a v e s h o w n t h a t t h e a p p e a r a n c e of t h e e n z y m e s y s t e m is co r r e l a t ed w i t h t h e a p p e a r a n c e of D N A s y n t h e s i s in vivo, b u t t h a t e n z y m a t i c a c t i v i t y c o n t i n u e s to inc rease o v e r t h e pe r iod 24 -48 h o u r s a f t e r h e p a t e c t o m y , w h e r e a s D N A s y n t h e s i s in vivo decreases .