The S-adenosyl- l-homocysteine (AdoHcy) hydrolases (SAHH) from Homo sapiens (Hs-SAHH) and from the parasite Trypanosoma cruzi (Tc-SAHH) are very similar in structure and catalytic properties but differ in the kinetics and thermodynamics of association and dissociation of the cofactor NAD (+). The binding of NAD (+) and NADH in SAHH appears structurally to be mediated by helix 18, formed by seven residues near the C-terminus of the adjacent subunit. Helix-propensity estimates indicate decreasing stability of helix 18 in the order Hs-SAHH > Tc-SAHH > Ld-SAHH (from Leishmania donovani) > Pf-SAHH (from Plasmodium falciparum), which would be consistent with the previous observations. Here we report the properties of Hs-18Pf-SAHH, the human enzyme with plasmodial helix 18, and Tc-18Hs-SAHH, the trypanosomal enzyme with human helix 18. Hs-18Tc-SAHH, the human enzyme with trypanosomal helix 18, was also prepared but differed insignificantly from Hs-SAHH. Association of NAD (+) with Hs-SAHH, Hs-18Pf-SAHH, Tc-18Hs-SAHH, and Tc-SAHH exhibited biphasic kinetics for all enzymes. A thermal maximum in rate, attributed to the onset of local structural alterations in or near the binding site, occurred at 35, 33, 30, and 15 degrees C, respectively. This order is consistent with some reversible changes within helix 18 but does require influence of other properties of the "host enzyme". Dissociation of NAD (+) from the same series of enzymes also exhibited biphasic kinetics with a transition to faster rates (a larger entropy of activation more than compensates for a larger enthalpy of activation) at temperatures of 41, 38, 36, and 29 degrees C, respectively. This order is also consistent with changes in helix 18 but again requiring influence of other properties of the "host enzyme". Global unfolding of all fully reconstituted holoenzymes occurred around 63 degrees C, confirming that the kinetic transition temperatures did not arise from a major disruption of the protein structure.