aging of imr-90 human diploid fibroblasts in vitro is accompanied by significant changes of polyamine metabolism , most notably , a 5-fold decrease of serum-induced activity of ornithine decarboxylase , the key enzyme in the biosynthesis of polyamines ( chen , k. y. , chang , z. f. , and liu , a. y.-c. ( 1986 ) j. cell . physiol . 129 , 142-146 ) .
C0016030
fibroblasts
C0007634
cell