Cellular uptake of succinylated catalase (Suc-CAT; Mw 227 kDa), bovine serum albumin (Suc-BSA; Mw 70 kDa), superoxide dismutase (Suc-SOD; Mw 34 kDa) and soybean trypsin inhibitor (Suc-STI; Mw 21 kDa) was studied using primary cultures of bovine brain microvessel endothelial cells (BMECs) developed as an in vitro model of the blood-brain barrier. Large succinylated proteins (Suc-CAT, Suc-BSA) were taken up by BMECs whereas significant uptake was not observed for native proteins and small succinylated proteins (Suc-SOD, Suc-STI). Uptake of Suc-BSA was significantly inhibited at 4 degrees C and in the presence of endocytosis inhibitors. Large succinylated proteins, maleylated BSA and dextran sulfate also showed competitive inhibition against Suc-BSA uptake while small succinylated proteins and carboxymethyl dextran did not show any effect. These results indicate that microvessel endothelial cells obtained from the brain endocytose succinylated proteins via a scavenger receptor-mediated mechanism for polyanions, and in addition, the importance of molecular weight or total numbers of anionic charges per one molecule of proteins is suggested. Usefulness of direct succinylation of proteins for their delivery to the brain capillary endothelium is thus demonstrated.