Serum stimulation of lysyl hydroxylase activity in cultured human skin fibroblasts.

In the absence of ascorbic acid, confluent human skin fibroblasts incubated in 0.5% serum-supplemented medium had one-third of the level of lysyl hydroxylase activity of cells incubated in media containing high serum concentrations (5-20%). This difference appeared to be due to a decline in the enzyme activity following serum deficiency, and was largely abolished by addition of ascorbic acid to the medium. The effect of serum deficiency was slow, manifesting in 48 h at the earliest, and was completely reversed by replenishing the medium with serum. Prolyl hydroxylase activity was independent of serum concentration, both in the absence and in the presence of ascorbic acid in the culture medium.