Abstract

A protein conjugate of α-amanitin was prepared by covalently linking an α-amanitin spacer derivative to bovine serum albumin with carbodiimide. This conjugate had an apparent K_I toward calf thymus DNA dependent RNA polymerase (RNA nucleotidyl transferase, E.C. 2.7.7.6) of 69 x 10^-9 M compared with an apparent K_I of 1.8 x 10^-9 M for free α-amanitin. The conjugate was 5.2, 2.5, and 0.52 fold more effective than free α-amanitin in inhibiting cell proliferation in human amnionic cells (AV₃), chinese hamster ovary cells (CHO), and mouse lymphocytic leukemia cells (EL4), respectively. Measurement of pinocytotic activity by uptake of [¹²⁵I] bovine serum albumin indicated that AV₃ cells were 3.6 fold more active than CHO cells whereas EL4 cells possessed negligible pinocytotic activity under the conditions studied. While differential susceptibility of the three cell lines to conjugated α-amanitin may be a function of differences in pinocytotic capability, conjugated α-amanitin is clearly more toxic to cultured cells than would be expected from its in vitro inhibition of calf thymus RNA polymerase II. These observations suggest the protein carrier of the conjugate may facilitate binding and possibly uptake of the bound amanitin by cells after which the amanitin may be cleaved, resulting in the inhibition of RNA synthesis and cell growth.