Loss of LDH activity during membrane filtration

Membrane filters rendered hydrophilic and composed of biphenyl polycarbonate, polyvinylidene fluoride (PVDF), acrylic copolymer, polysulfone, and mixed esters of cellulose were evaluated to determine which type of filter best can be used for the filtration of lactate dehydrogenase (LDH) solution. Also, the effect of the membrane pore size was evaluated. LDH solution was passed through the filters at a controlled flow rate, after which the filtrate was assayed for LDH activity and protein content using the Bradford method. Polycarbonate and PVDF filters generally showed low loss of protein, except 5 microns PVDF filters. Mixed esters of cellulose, acrylic copolymer, and polysulfone caused considerable loss of protein during passage of the LDH solution through the filter. Interestingly, it was also found that, generally, as the pore size increased the amount of protein loss decreased. However, 5 microns PVDF and mixed esters of cellulose filters showed more loss than their corresponding 0.65 microns and 3 microns pore size filters, respectively. In all cases, more protein was recovered in the filtrate fractions as the volume of LDH solution filtered was increased, suggesting that the mechanism of loss is adsorption and that the magnitude of loss is related to saturation of the matrix polymer surfaces.