Lactoferrin (Lf)t is an 80-kD iron-binding glycoprotein found in high concentrations in human milk and at much lower but detectable levels in a number of other secretions of glandular epithelium (1). The principal functions of Lf are thought to be iron transport and storage (2), and bacteriostasis through strong chelation of iron required for microbial growth (3). However, Lf is also a major constituent of the secondary or specific granules of neutrophils (4), and has been implicated in several operational and regulatory functions in the immune and hematopoietic systems (5). To date, only a single form of Lf has been described, and is presumed to account for all of the diverse functions of the molecule.One of us (M. R. Das) previously reported (6) the presence in human milk of a unique and potent RNase activity, termed human milk RNase (hmRNase). This enzyme was initially identified based on its interference with the detection ofretrovirallike RNAs in human milk, and was subsequently shown to be present in low concentrations in a consanguineous community with a high incidence of breast cancer, the Parsi women of Bombay. The hmRNase was detected in high concentrations only in human milk and was thus considered to be potentially a marker for breast epithelium as well as for risk to development of breast cancer.The hmRNase has recently been purified to homogeneity and revealed to be a high molecular mass (80 kD) glycoprotein with a preference for mRNAs, viral RNAs, and purine homopolymers (7). Activity of the enzyme is influenced by various cations and is optimal at pHs of 7.5-8 .0.We report here that hmRNase is an isoform of Lf, sharing physical, chemical, and antigenic properties with the major species of Lf, but differing from it in the possession of potent nuclease activity and in the lack of significant iron-binding capacity. These findings establish the presence of multiple forms of Lf, with very distinctive properties, that may be related to the highly diverse physiological functions of the molecule .

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Definitive evidence for the occurrence of cell fusion in tumorigenesis was sought in methylcholanthrene-induced sarcomas. This was approached by using allophenic mice generated from strains differing for electrophoretic variants of the ubiquitous, dimeric enzyme glucose phosphate isomerase, with fusion assessed by heterodimer formation. Eight-three carefully trimmed primary tumor samples (from 23 individual tumors in allophenic mice) were analyzed, as were 1,140 clones derived from them. In all primary tumor samples, zymograms exhibited one GPI homopolymeric band. Expression of a hybrid band (indicative of a fusion event) was not observed in these samples. However, 9 (0.8%) of the tumor clones demonstrated a distinct and reproducible hybrid band which was uniformly lost upon recloning. Our data suggest that cell fusion, although uncommon, occurs in the clonogenic cell fraction during primary MCA tumorigenesis and is followed rapidly by chromosome segregation.

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