Pyridoxylated adult human hemoglobin (HbAo) was prepared using a one molar equivalent of pyridoxal5-phosphate (PLP) per heme and reduced with either NaCNBH, or NaBH,. A separate sample was pyridoxylated and passed through a mixed-bed ion exchange column without reduction. All three preparations had a P5" of 29 2 tom and a cooperativity of n = 2.4 + 0.1. These preparations, in both the oxy and deoxy forms, were then treated with 7 equivalents of glutaraldehyde per tetramer at pH 6.8 at 4Β°C and at room temperature. The polymerization invariably reduced the Ps0 to 18 k 2 torr with Hill coefficients of less than 2. These solutions, with or without further reduction using NaCNBH,, all retained the PLP in differing amounts (2-3 moles/tetramer). Methemoglobin concentrations were increased during the polymerization reaction. The normal pyridoxylation procedure, using sodium borohydride reduction, resulted in a number of different molecular species. Polymerization with glutaraldehyde caused a further proliferation of molecular species that could not be separated by anion exchange chromatography or by isoelectric focusing. The extent of polymerization, estimated by gel exclusion chromatography and SDS polyacrylamide gel electrophoresis, was from 40 to 50%. Analysis of the reverse phase chromatograms, which separate the heme and the aand P-chains, showed extensive polymerization and distribution of the radioactively labeled PLP on the protein for all preparations. All of the polymerized and pyridoxylated samples were unstable, and showed different chromatographic patterns after storage at 4Β°C for 1 month. Attempts to stabilize these preparations by further reduction with NaCNBH,, gave products with a lower 9," and lower cooperativity. When the reactions were conducted with a purified HbAo, heterogeneity was somewhat decreased compared to the normally used stroma-free hemoglobin, but a large number of molecular species were still formed.