Differences in the subcellular localization of the stimulatory effects of glucose and pyruvate on protein synthesis in rat thymus cells in vitro.

Abstract

AbstractThymocytes incubated as cell suspensions in vitro are known to be markedly dependent upon added glucose for maintenance of maximal rates of incorporation of radiolabelled amino acids into protein. This requirement is only partially satisfied by other added substrates, such as pyruvate. Evidence is presented that incorporation of amino acids into protein associated with the nuclear fraction isolated from these cells is more dependent upon added glucose than is labelling of protein found in the rest of the cell.The dependence of the labelling of nuclear protein upon glucose is shown by comparing the ability of glucose and pyruvate to stimulate the incorporation of [14C‐L] valine into the protein of nuclear and cytoplasmic fractions of thymus cells. The fractions are isolated on sucrose gradients after incubating suspensions of cells in substrate‐free medium for two hours, adding carbohydrates and labelled L‐valine for 30 min and then stopping the incubation by breaking the cells with hypotonic shock. When the protein‐synthetic stimulatory effects of glucose and pyruvate are compared, glucose is almost equally capable (90%) at stimulating rates of protein synthesis in nuclear compared to cytoplasmic fractions. Pyruvate is much less effective in nuclear than in cytoplasmic fractions (30%).Evidence is also presented from pulse‐chase experiments that the glucosedependent labelling of protein associated with the nuclear fraction occurs within that fraction, as opposed to migration to the nuclear fraction after being synthesized elsewhere.It is suggested from these and other data that a unique ability of glucose to provide non‐mitochondrial ATP to the nucleus may be central to the dependence of the labelling of nuclear protein on this substrate.