Nitric oxide synthase activity in molluscan hemocytes

Area of Science:

• Biochemistry
• Immunology
• Invertebrate Zoology

Background:

• Molluscan hemocytes are key players in invertebrate immune responses.
• Nitric oxide synthase (NOS) is vital in vertebrate immune and signaling pathways.
• The presence and function of NOS in molluscan hemocytes remain largely uncharacterized.

Purpose of the Study:

• To biochemically demonstrate and characterize nitric oxide synthase (NOS) activity in the hemocytes of the freshwater snail Viviparus ater.
• To investigate the properties and regulation of this enzyme in molluscan immune cells.

Main Methods:

• Enzyme assays measuring [3H]citrulline conversion and nitrite/nitrate formation.
• Investigating cofactor dependency (NADPH) and inhibition by NG-monomethyl-L-arginine.
• Determining kinetic parameters (Km for L-arginine) and calcium dependency.
• Assessing the effect of lipopolysaccharide (LPS) stimulation on NOS activity.
• Fractionation of hemocyte lysates and immunological inhibition using anti-rat NOS antiserum.

Main Results:

• NOS activity was confirmed in Viviparus ater hemocytes via [3H]citrulline and nitrite/nitrate production.
• The enzyme is NADPH-dependent, with a Km of 2.5 microM for L-arginine and sensitive to mammalian NOS inhibitors.
• Significant activity (70%) was observed at very low free Ca2+ concentrations (3 nM).
• LPS treatment elevated total NOS activity by 2.4-fold.
• Immunological assays using antiserum against rat cerebellar NOS confirmed the enzyme's identity.

Conclusions:

• This study provides the first biochemical evidence for NOS activity in molluscan hemocytes.
• The characterized NOS enzyme in snail hemocytes shares properties with mammalian NOS, suggesting conserved functions in immune defense.
• These findings highlight the role of nitric oxide signaling in invertebrate immunity.