Catalyzes the reversible reduction of (S)-acetoin to (S,S)-butane-2,3-diol (L-BD) in the presence of NADH. To a lesser extent, can also catalyze the irreversible reduction of diacetyl to (S)-acetoin. Cannot oxidize meso-BD, D-BD, 2-butanol, 1,2-propanediol, ethanol, acetol, 1,2-butanediol, 1,3-butanediol, n-butanol, and n-propanol. Cannot reduce (R)-acetoin, acetol, dihydroxyacetone and 2,4-pentanedione. (S,S)-butane-2,3-diol + NAD(+) = (S)-acetoin + H(+) + NADH (S)-acetoin + NAD(+) = diacetyl + H(+) + NADH Slightly activated by Ba(2+), Ca(2+), Mn(2+), Mg(2+), and Co(2+), while Hg(2+) and Cu(2+) cause marked inhibition of the activity. Ni(2+), Zn(2+) and Cd(2+) have no effect on the catalytic activity. Is also slightly inhibited by lactate, pyruvate, succinate, acetate and formate. Optimum pH is 10.5 for the oxidative reaction with L-BD and 6.0 for the reductive reaction with (S)-acetoin. Stable from pH 7.0 to 9.0. Stable up to 37 degrees Celsius but is rapidly inactivated over 40 degrees Celsius. Homotetramer. Belongs to the short-chain dehydrogenases/reductases (SDR) family.