Table 2.

Estimated M_rof components of mouse pre-pro-IGF-I

	No. of residues	Estimated mass (kDa)
Pre-pro-IGF-IEb	159	17.92
Pro-IGF-IEb	111	12.57
Mature IGF-I	70	7.68
Eb peptide	41	4.89
Exon 4 extension	16	1.95
MGF	25	2.94

Number of residues corresponding to each component of pre-pro-IGF-I were determined from NCBI Reference Sequence NP_034642.2 [IGF-I isoform 1 pre-proprotein (Mus musculus)] (11). Estimated average molecular masses were determined by computing mass of peptides with hydrogen N-terminal and free acid C-terminal groups. Although the Eb peptide represents 41 residues of the pre-pro-IGF-IEb sequence, its cleavage from mature IGF-I likely produces a 40-amino-acid peptide due to cleavage at Arg⁷¹. Cleavage of pro-IGF-I yields a 71-amino-acid peptide that contains the 70-residue mature IGF-I as well as Arg⁷¹ as the most C-terminal residue. Arg⁷¹ is subsequently removed, presumably by carboxypeptidase activity, thus yielding mature IGF-I and the Eb peptide.