title: Site-specific folate conjugation to a cytotoxic protein.
abstract: Conjugation to folic acid is known to enhance the uptake of molecules by human cells that over-produce folate receptors. Variants of bovine pancreatic ribonuclease (RNase A) that have attenuated affinity for the endogenous ribonuclease inhibitor protein (RI) are toxic to mammalian cells. Here, the random acylation of amino groups in wild-type RNase A with folic acid is shown to decrease its catalytic activity dramatically, presumably because of the alteration to a key active-site residue, Lys41. To effect site-specific coupling, N(δ)-bromoacetyl-N(α)-pteroyl-l-ornithine, which is a folate analogue with an electrophilic bromoacetamido group, was synthesized and used to S-alkylate Cys88 of the G88C variant of RNase A. The pendant folate moiety does not decrease enzymatic activity, enables RI-evasion, and endows toxicity for cancer cells that over-produce the folate receptor. These data reveal a propitious means for targeting proteins and other molecules to cancer cells.
authors.affiliation: Department of Biochemistry, University of Wisconsin-Madison, 433 Babcock Drive, Madison, WI 53706, USA.
authors.forename: Bryan D
authors.initials: BD
authors.lastname: Smith
authors.forename: Joshua J
authors.initials: JJ
authors.lastname: Higgin
authors.forename: Ronald T
authors.initials: RT
authors.lastname: Raines
mesh_terms: D000215:Acylation; D000818:Animals; D002384:Catalysis; D020134:Catalytic Domain; D002417:Cattle; D005492:Folic Acid / Q000737:chemistry*; D020128:Inhibitory Concentration 50; D013058:Mass Spectrometry; D010926:Placental Hormones / Q000737:chemistry*
chemical_list: D010926:Placental Hormones; C022052:placental ribonuclease inhibitor; D005492:Folic Acid