Purification to homogeneity and characterization of nonproteolyzed potato (Solanum tuberosum) tuber hexokinase 1

We have developed an extraction procedure that improves the stability of potato (Solanum tuberosum L.) tuber hexokinase (HK) after extraction. Using this protocol, we showed that at least four HK isoforms are present in this tissue, and they can be separated by hydrophobic-interaction chromatography on a butyl-Sepharose (TM) 4 Fast Flow column. One of the main HK isoforms was purified to homogeneity using further chromatographic separations on red dye, DEAE Fractogel, hydroxyapatite, cibacron blue, and MonoQ matrices. HK-specific activity of this fraction (10.2 U.mg protein(-1)) corresponds to an enrichment of more than 5500-fold, with a yield of 0.9%. This is the highest reported HK-specific activity from a plant source. The purified enzyme consisted of a monomer with a subunit apparent M-r of 51 kDa when analyzed by SDS-PAGE. This polypeptide was recognized by affinity-purified anti-Solanum chacoense Bitt. recombinant HK IgGs. The protein was digested with trypsin and its digestion products were subjected to MS - MS sequencing after HPLC separation. The sequences of these tryptic peptides matched the predicted coding sequence of the S. tuberosum HK1 gene with a coverage of 57%. Examination of the kinetic properties of the purified protein HK1 indicates that it may be regulated by the internal O-2 concentration of the tuber because of its sensitivity to acidic pHs and inhibition by ADP.