Properties and developmental regulation of polysialyltransferase activity in the chicken embryo brain

The properties and developmental regulation of vertebrate polysialyltransferase (PST), an enzyme activity responsible for extension of α2,8-linked sialic acid homopolymers (PSA) associated with the fifth Ig domain of the neural cell adhesion molecule (NCAM), have been studied. The assay for PST used exogenous NCAM as a substrate, with a PSA-specific endoneuraminidase as a control for specificity. Optimal conditions for PST activity at 37 °C were found to be pH 6.0 in the presence of divalent cations (Mn²⁺, 20 mM). The enzyme K(m) was found to increase with increasing polymer length, ranging from 0.7 to 0.07 μM. The developmental regulation both of PST activity and of the addition of PSA to NCAM were studied in chick whole brain, tectum, and cerebellum and found to be precisely coordinated. In each tissue PSA and PST were highest during early stages of morphogenesis, followed by a decrease as development reached completion. The insertion of the VASE exon in the fourth Ig domain of NCAM was also found to parallel closely the developmental down-regulation of PSA, and on this basis could be considered a potential determinant in the specific polysialylation of NCAM. However, in direct tests of this hypothesis in transfected cells the presence of VASE did not markedly alter the level of NCAM polysialylation or alter the affinity of PST for the NCAM substrate.