Previous studies have shown that murine macrophages immunostimulated with interferon gamma and Escherichia coli lipopolysaccharide synthesize NO2-, NO3-, and citrulline from L-arginine by oxidation of one of the two chemically equivalent guanido nitrogens. The enzymatic activity for this very unusual reaction was found in the 100,000g supernatant isolated from activated RAW 264.7 cells and was totally absent in unstimulated cells. This activity requires NADPH and L-arginine and is enhanced by Mg2+. When the subcellular fraction containing the enzyme activity was incubated with L-arginine, NADPH, and Mg2+, the formation of nitric oxide was observed. Nitric oxide formation was dependent on the presence of L-arginine and NADPH and was inhibited by the NO2-/NO3- synthesis inhibitor NG-monomethyl-L-arginine. Furthermore, when incubated with L-[guanido-15N2]arginine, the nitric oxide was 15N-labeled. The results show that nitric oxide is an intermediate in the L-arginine to NO2-, NO3-, and citrulline pathway. L-Arginine is required for the activation of macrophages to the bactericidal/tumoricidal state and suggests that nitric oxide is serving as an intracellular signal for this activation process in a manner similar to that very recently observed in endothelial cells, where nitric oxide leads to vascular smooth muscle relaxation [Palmer, R. M. J., Ashton, D. S., & Moncada, S. (1988) Nature (London) 333, 664-666].

Enzymatic browning is a major factor contributing to quality loss in foods and beverages. Sulfiting agents are used commonly to control browning; however, several negative attributes associated with sulfites have created the need for functional alternatives. Recent advances in the development of nonsulfite inhibitors of enzymatic browning are reviewed. The review focuses on compositions that are of practical relevance to food use.

The macrophage cell line RAW 264.7 when activated with Escherichia coli lipopolysaccharide and interferon-y synthesized nitrite (NO-) and nitrate (NO-). Medium change after the activation showed that L-arginine was the only amino acid essential for this synthesis. D-Arginine would not substitute for L-arginine. Other analogues that could replace L-arginine were L-homoarginine, L-arginine methyl ester, Larginamide, and the peptide L-arginyl-L-aspartate. L-Argininic acid, L-agmatine, L-ornithine, urea, L-citrulline, and ammonia were among the nonprecursors, while L-canavanine inhibited this L-arginine-derived NO /NO-synthesis. When morpholine was added to the culture medium of the activated RAW 264.7 macrophages, N-nitrosation took place, generating Nnitrosomorpholine. GC/MS experiments using L-[guanido-15N2]arginine established that the NO /NO-and the nitrosyl group of N-nitrosomorpholine were derived exclusively from one or both of the terminal guanido nitrogens of arginine. Chromatographic analysis showed that the other product of the L-arginine synthesis of NO /NO-was L-citrulline. The role of the respiratory burst in NO /NO-synthesis was examined using the macrophage cell lines J774.16 and J774 C3C. Both cell lines synthesized similar amounts of NO-/NO-. However, J774 C3C cells do not produce superoxide and hence do not exhibit the respiratory burst. Additional experiments also ruled out the involvement of the respiratory burst in NO -/NO synthesis.We have previously reported that primary cultures of murine macrophages, when treated with Escherichia coli lipopolysaccharide (LPS), synthesize . The addition of T lymphocytes to macrophage cultures enhanced this synthesis (1) and was due primarily to the T-cell-derived lymphokine interferon-y (IFN-y) (2). Regardless of the stimulant used, a time lag of 6-12 hr was observed for NOj/NOsynthesis (2, 3), during which protein synthesis required for the response occurred (D.J.S. and M.A.M., unpublished observations). NO-and NO-are stable in the presence of macrophages in culture and do not inhibit additional synthesis (2). When NOj was added to macrophage cultures, it was not oxidized to NOj, suggesting that the NO-synthesized is not derived from NO-(2). When secondary amines, such as morpholine, were added to cell culture medium of activated macrophages synthesizing NO-/NO-, it was found that the macrophages also carry out N-nitrosations, generating carcinogenic N-nitrosamines (4). Results from the nitrosamine synthesis studies showed that NO-was not the nitrosating agent but suggested that the amines were reacting with an intermediate in the pathway from the precursor to NOj (4).Although the toxicity of NO-to some bacteria is well established (5), until very recently the biological role for NO /NO-synthesis was unknown. Previous studies suggested a link between NO /NO-synthesis and the acquisition of increased nonspecific bacterial resistance (1). In addition, Hibbs et al. have reported that L-arginine is required for the selective metabolic inhibition of tu...

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