Phosphate content of mammalian neurofilaments

The phosphate content of neurofilaments from bovine spinal cord and of their constituent polypeptides was determined. Neurofilament preparations, as isolated, contained substantial amounts of phosphate, having between 65 and 98 nmol of phosphate/mg of protein. When the polypeptides of the neurofilament were assayed for phosphate individually, the 210,000-and 160,000-dalton polypeptides were found to possess large numbers of phosphate groups, while the 68,000-dalton polypeptide had less than 1 mol of phosphate/mol of polypeptide. The 210,000-dalton polypeptide had about 104 mol of phosphate bound/mol of polypeptide, and the 160,000-dalton polypeptide had about 26 mol of phosphate/mol of polypeptide. This distribution of phosphate among the polypeptides of the neurofilament preparation was qualitatively confirmed when sodium dodecyl sulfate-polyacrylamide gels of the preparations were stained for phosphate. Bands corresponding to the 210,000- and 160,000-dalton polypeptides were readily detected. No other phosphate-stained bands were seen, even on heavily loaded gels.