The pH dependence of borohydride as an aldehyde reductant.

The aldehyde-reducing capacity of borohydride has been investigated in the sequence periodic acid-borohydride-periodic acid-Schiff and variants. Densitometric studies on rat colonic mucins show that borohydride incompletely blocks periodate-engendered aldehydes unless the pH is above 8.2. Below this value, some aldehydes are not reduced and continue to be Schiff-stainable, while others are subsequently gerenated by the second exposure to periodic acid. The effect is more pronounced in paraffin than in cryostat sections, but does not apply to human colonic mucins.