Competitive antagonist of atrial natriuretic peptide NPR-A receptors. It binds competitively to atrial natriuretic peptide (ANP) receptors from bovine adrenal cortex and inhibits the ANP-induced intracellular cGMP accumulation in bovine aorta smooth muscle cells. Can also reduce the increase in cGMP produced by ANP or BNP (brain natriuretic peptide) in pregnant guinea pig myometrium, but has no effect on relaxation induced by either peptide. Blocks the ANP-induced human sperm acrosome reaction without affecting the acrosomal exocytosis or sperm motility. Also exhibits major and deleterious nonspecific (and even cytotoxic) effects on human fat cells, as it displays noncompetitive antagonism and exerts an inhibitory action on basal and beta-adrenergic receptor-induced lipolytic response. Anantin has been tested for antimicrobial activity against a broad variety of bacteria and fungi, but no antibiotic activity has been found. Inhibits the binding of ANP by 50% (IC(50)) at 1.0 uM. The isopeptide linked residue 8 is shown as Asn rather than Asp as mentioned in PubMed:1826288, because it is not known whether Asp or Asn is encoded and the isopeptide bonds are almost always formed between the amides Asn or Gln and N6-lysine or alpha amino groups, with the liberation of an ammonia that makes the reaction essentially irreversible.