Evidence for a contribution of paramagnetic ions to water proton spin‐lattice relaxation in normal and malignant mouse tissues

Paramagnetic ions complexed to proteins may lose, retain, or enhance solvent paramagnetic relaxation (SPR) relative to free solution. We measured T₁ and T₂ of three mouse cancers, their normal counterparts, and six additional tissues. Long T₁ of cancers was not caused by necrosis or by different contents of water, fat, or blood. Dissociable (TCA‐extractable) and nondissociable (ashed) Mn, Cu, and Fe were measured by AA. Cancers had less Mn, Cu, and Fe than did normal counterparts. All 12 tissues had inverse correlations between T₁ and dissociable Mn and Cu. For Mn alone to account for reduced T₁, the extent to which SPR of the Mn‐protein complexes would be enhanced is by factors of 0.6 to 13, below the maximum observed in Mn‐enzymes. Different amounts of paramagnetic ion‐protein complexes may account for part of the differences in T₁ of water protons in different tissues, and the longer T₁ of cancer cell water may be caused in part by reduced amounts Of such complexes. © 1991 Academic Press. Inc.