14C-labeled substrate catabolism by human diploid fibroblasts derived from infants and adults

Untransformed diploid skin fibroblasts from eight normal adults, aged 24 to 74 years, catabolized several ¹⁴C-labeled substrates less effectively than cells from ten normal male infants. ¹⁴C-labeled substrate metabolism was quantitated either by measuring the evolution of ¹⁴CO₂ from the ¹⁴C-labeled compounds or the incorporation of ¹⁴C into cellular protein via transamination of tricarboxylic acid cycle intermediates derived from the ¹⁴C-labeled substrates. With these methods, adult cells catabolized [1-¹⁴C]butyrate, [1-¹⁴C]octanoate, and 1-[2-¹⁴C]leucine at rates 44 to 64% of those found in infant cells. The oxidation of [1,4-¹⁴C]succinate and [U-¹⁴C]malate was identical in both infant and adult cells, while [2,3-¹⁴C]succinate catabolism was mildly decreased in adult cells (65–80% of control). These observations parallel those made in rat tissues and confirm that the same phenomenon occurs in cultured human fibroblasts.