Purification of human galactokinase (GALK). Different amounts of Ni2+-NTA affinity-purified GALK protein were loaded on sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) as follows: M, molecular weight markers; 1, unloaded; 2, 1.25 μg GALK; 3, unloaded but some spillover of GALK from lane 4 was seen; 4, 6.25 μg GALK.