this is based on the assumption that if whole , viable cells can catalyze the transfer of sialic acid from cmp-sialic acid to endogenous acceptors , then the transferases carrying out the reaction must be on the cell surface , provided that ( 1 ) cmp-sialic acid does not enter the cells , and ( 2 ) cmp-sialic acid does not break down outside the cells , yielding free sialic acid which then may enter the cells , in amounts large enough to explain the incorporation .
C0699040
cell surface
C0007634
cells