This paper reports the isolation and characterization from bovine milk of two 
proteins: angiogenin-1, a recently discovered angiogenin, and lactogenin, a 
novel protein. Both proteins were adsorbed on and eluted closely from 
CM-Sepharose and Mono S. Lactogenin possessed a molecular weight (17 kDa) 
slightly higher than that of angiogenin-1 (15 kDa). Lactogenin had a higher 
ribonucleolytic (RNase) activity than angiogenin-1 towards yeast transfer RNA 
(tRNA). The Km values estimated for the RNase activities of angiogenin-1 and 
lactogenin were 51 microM and 40 microM respectively. Both were specific for 
poly C. The optimal pH for the RNase activities of angiogenin-1 and lactogenin 
was 7.75 and 7.5 respectively. Comparison of the amino acid sequences of 
cyanogen bromide fragments and the pyroglutaminase-treated N-terminal fragment 
of lactogenin with the sequence of bovine liver RNase (RNase BL4) revealed 
identity in residues 3-22, 24, 26-27, 37, 41-44, 46-50, 54, 56, 63, 72-80, and 
83. Considerable similarity to the N-terminal sequence of angiogenin-2 was also 
noted. Both lactogenin and angiogenin-1 inhibited cell-free translation in a 
rabbit reticulocyte lysate system with an IC(50) below 100 nM.