Solvent exchange and ligand substitution reactions of transition metal ions and complexes form the basis for understanding many chemical and biochemical processes that involve the interaction with metal ions and complexes. For this reason, much fundamental research has been devoted to reveal the intimate nature of solvent exchange and ligand substitution processes.1 The application of high-pressure kinetic techniques has played a crucial role in the t' cidation of the different mechanisms according to which such p, ocesses can occur, ranging from limiting dissociative (D) to interchange (Id and Ia) and limiting associative (A) mechanisms.2-7 Symmetrical solvent and ligand exchange reactions and nonsymmetrical solvent and ligand displacement reactions have been demonstrated to exhibit very characteristic volumes of activation. Such data are presently available for solvent exchange and complex formation reactions of the divalent firstrow transition metal ions V2+, Mn2+, Fe2+, Co2+, and Ni2+ and demonstrate a gradual changeover in mechanism from Ia for the larger metal ions (V2+ and Mn2+) to Id for the smaller metal ions (Fe2+, Co2+, and Ni2+) in aqueous medium.* 123•6 Although it is of fundamental importance to have similar data for solvent exchange reactions of Cr2+ and Cu2+, such measurements are complicated by Jahn-Teller distortions and associated relaxations. Á recent 170 NMR study8 shows that the rate of inversion of the Jahn-

Leukocytes derived from the peripheral blood of psoriatic patients demonstrated an enhanced chemotactic response compared with leukocytes from healthy subjects. No significant difference was detected between the chemotactic response of leukocytes from patients with minimal or no skin involvement and those from patients with extensive lesions. Psoriatic leukocytes also had a significantly higher capacity to engulf 125I labeled Shigella flexneri than control leukocytes. It is postulated that a decrease in the cyclic AMP/cyclic GMP ratio in psoriatic leukocytes, similar to the imbalance of these 2 cyclic nucleotides found in the lesional epidermis of psoriasis, might be the cause of their enhanced chemotactic and phagocytic activities.

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