The in vivo disposition characteristics of some aconitylated proteins in mice were studied after intravenous injection in relation to their molecular properties such as overall negative charge and size of the molecules. Superoxide dismutase (SOD, M-w = 32 000) and bovine serum albumin (BSA, M-w = 67 000) were used to produce aconitylated derivatives with a different extent of modification. Aconitylated SOD (Aco-SOD) was only moderately taken up by the liver in spite of its negative charge density, whereas aconitylated BSA (Aco-BSA) with a smaller charge density was taken up by the liver very efficiently. Aco-BSA was more rapidly cleared by the liver than succinylated BSA due to the introduction of more anionic groups, especially when the degree of modification was low. Interestingly, highly aconitylated BSAs exhibited significant accumulation in the kidney at higher doses, especially when the hepatic uptake was saturated. Further analysis that was based on a physiological pharmacokinetic model including a saturable hepatic uptake process revealed that the higher the number of negative charges on the proteins, the higher was the apparent affinity of aconitylated proteins for the hepatic SRs. In general, the affinity of aconitylated proteins was higher than that of succinylated proteins when the degree of acylation was the same. Thus, the present study indicates that apart from charge density on the proteins the molecular size of the proteins is important for SR-mediated uptake in the liver. Aconitylation of proteins seems more suitable than succinylation for targeting of proteins to the liver nonparenchymal cells, in particular, at a low degree of acylation of the proteins at which the enzymatic activity is better retained for sufficient negative charges introduced. (C) 2003 Elsevier Science B.V. All rights reserved.