Catalyzes the oxidation of L-fucose to L-fuconolactone in the presence of NADP(+). Also active against L-galactose and, to a much lesser degree, D-arabinose. Uses NADP(+) as a hydrogen acceptor much more efficiently than NAD(+). a D-threo-aldose + NAD(+) = a D-threo-aldono-1,5-lactone + H(+) + NADH Inhibited strongly by Hg(2+), Cd(2+) and para-chloromercuribenzoic acid (PCMB) and weakly by Zn(2+) and iodoacetamide. Also inhibited strongly by L-xylose but not D-glucose. Exhibits a shoulder at 289 nm, but the characteristic absorption spectrum caused by prosthetic group is not observed. Optimum pH is 9-10.5. Optimum temperature is 37-40 degrees Celsius. Activity decreases at 50 degrees Celsius. Belongs to the aldo/keto reductase family.