Cell‐Cycle Dependence of a Ganglioside Glycosyltransferase Activity and Its Inhibition by Enkephalin in a Neurotumor Cell Line

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Abstract
Rat glioma mouse neuroblastoma hybrid neurotumor cells (NG108‐15), synchronized by amino acid deprivation, showed a cell‐cycle‐dependent peak of activity of a ganglioside N‐acetylgalactosaminyl transferase 14‐24 h following release from the cell cycle block (S/G2 phase). Maximal expression of two typical lysosomal hydrolases, N‐acetyl‐β‐hexosaminidase and β‐galactosidase, occurred between 18 and 21 h following release (S phase), declining to G1 phase levels during the peak of N‐acetylgalactosamine (GalNAc) transferase activity. In addition, glycosyltransferase activity in G2 phase cells showed an increase in apparent Vmax (suggesting the presence of more enzyme/mg of cell protein) and apparent binding affinity for uridine diphosphate N‐acetylgalactosamine (UDP‐GalNAc) (32 versus 14 M) when compared to transferase activity in the G1 phase. However, the opioid peptide enkephalin [D‐Ala2, o‐Leu5], which inhibits ganglioside GalNAc transferase activity in unsynchronized NG108‐15 cultures, was much more inhibitory in whole cells 8 h after release from the cell cycle block (G1 phase) than in cells 20 h after release (G, phase), with 50% inhibition occurring at 2 ± 10‐9M and 2 ± 10‐7M, respectively. These results suggest that the GalNAc transferase activity is regulated in more than one way during the cell cycle, since both Vmax and Km changes are observed, and that the cyclic AMP‐dependent mechanism by which opiates reduce transferase activity is receptor mediated and cell cycle dependent.