Down-Regulation of Porcine Heart Diaphorase Reactivity by Trimanganese Hexakis(3,5-Diisopropylsalicylate), Mn3(3,5-DIPS)6, and Down-Regulation of Nitric Oxide Synthase Reactivity by Mn3(3,5-DIPS)6 and Cu(II)2(3,5-DIPS)4

Purposes of this work were to examine the plausible down-regulation of porcine heart diaphorase (PHD) enzyme reactivity and nitric oxide synthase (NOS) enzyme reactivity by trimanganese hexakis(3,5-diisopropylsalicylate), [Mn3(3,5-DIPS)6] as well as dicopper tetrakis(3,5- diisopropylsalicylate, [Cu(II)2(3,5-DIPS)4] as a mechanistic accounting for their pharmacological activities. Porcine heart disease was found to oxidize 114 μM reduced nicotinamide-adenine- dinucleotide-′3-phosphate (NADPH) with a corresponding reduction of an equivalent concentration of 2,6-dichlorophenolindophenol (DCPIP). As reported for Cu(II)2 (3,5-DIPS)4, addition of Mn3(3,5-DIPS)6 to this reaction mixture decreased the reduction of DCPIP without significantly affecting the oxidation of NADPH. The concentration of Mn3(3,5-DIPS)6 that produced a 50% decrease in DCPIP reduction (IC50) was found to be 5μM. Mechanistically, this inhibition of DCPIP reduction with ongoing NADPH oxidation by PHD was found to be due to the ability of Mn3(3,5-DIPS)6 to serve as a catalytic electron acceptor for reduced PHD as had been reported for Cu(II)2(3,5-DIPS)4. This catalytic decrease in reduction of DCPIP by Mn3(3,5-DIPS)6 was enhanced by the presence of a large concentration of DCPIP and decreased by the presence of a large concentration of NADPH, consistent with what had been observed for the activity of Cu(II)2(3,5-DIPS)4 Oxidation of NADPH by PHD in the presence of Mn3(3,5-DIPS)6 and the absence of DCPIP was linearly related to the concentration of added Mn3(3,5-DIPS)6 through the concentration range of 2.4 μM to 38μM with a 50% recovery of NADPH oxidation by PHD at a concentration of 6 μM Mn3(3,5-DIPS)6 Conversion of [3H] L-Arginine to [3H] L-Citrulline by purified rat brain nitric oxide synthase (NOS) was decreased in a concentrated related fashion with the addition of Mn3(3,5-DIPS)6 as well as Cu(II)2(3,5-DIPS)4 which is an extention of results reported earlier for Cu(II)2(3,5-DIPS)4. The concentration of these two compounds required to produce a 50% decrease in L-Citrulline synthesis by NOS, which may be due to down-regulation of NOS, were 0.1 mM and 8μM respectively, consistent with the relative potencies of these two complexes in preventing the reduction of Cytochrome c by NOS. It is concluded that Mn3(3,5-DIPS)6, as has been reported for Cu(II)2 (3,5-DIPS)4 , serves as an electron acceptor in down-regulating PHD and both of these complexes down-regulate rat brain NOS reactivity. A decrease in NO synthesis in animal models of seizure and radiation injury may account for the anticonvulsant, radioprotectant, and radiorecovery activities of Mn3(3,5-DIPS)6 and Cu(II)2(3,5-DIPS)4.


Introduction
In neuronal cells of the brain [1] and many other cell types of other tissues [2] nitric oxide (.N=O, NO) and -Citrulline (-Cit) are produced by constitutive cytosolic reduced nicotinarnideadenine-dinucleotide-3'-phosphate (NADPH)-dependent nitric oxide synthase (NOS) oxidation of -Arginine (-Arg) at a reductase-oxygenase coupled domain. Roles of NO have been reviewed by Moncada et al. [2], Moncada [3], Lancaster [4], Meller and Gebhart [5], and Nussler and Billiar [6]. It was pointed out by Moncada et al. [2] that as early as 1977 Miki et al. [7] suggested that the role Hexaakis-(3, 5-Diisopropylsalicylate), of NO in normal brain function was activation of cytosolic guanylyl cyclase (GC). This activation by NO is now recognized as a feature of its role as a retrograde neurotransmitter [8] that has most recently been well characterized as a signal transduction mechanism within and between different areas of the brain [9]. Inducible NOS activity is also important in immune-mediated white blood cell responses to inflammation, cancer, and infection and cytokine mediation of many organ inflammatory diseases as well as pain [2,10].
Dicopper(II) tetrakis(3,5-diisopropylsalicylate), [Cu(II)2(3,5-DIPS)4], has been shown to have antiinflammatory, antiulcer,, anticonvulsant, anticancer, anticarcinogenic, antimutagenic, antidiabetic, analgesic, radioprotectant, and radiorecovery activities and decreases ischemiareperfusion injury [11 and references therein]. Following the report that Cu(II)z(3,5-DIPS)4 inhibits NADPH-dependent mixed function oxidase systems by serving as an electron acceptor, it was studied in the NADPH-dependent porcine heart diaphorase (PHD) system and in a NADPHdependent tissue staining procedure used to detect NOS activity [10]. These studies revealed that Cu(II)2(3,5-DIPS)4 down-regulated PHD wherein PHD continued to oxidize NADPH, without reducing an obligate electron acceptor, dichlorophenolindophenol (DCPIP). It was also found that reducing equivalents derived from NADPH by PHD reduced Cu(II) to Cu(I) in a catalytic fashion causing a decrease in the reduction of DCPIP. These results lead to the provision of indirect evidence that the synthesis of NO by NOS in brain tissue sections was also decreased by incubating these tissue sections in medium containing Cu(II)2(3,5-DIPS)4 [10].
Either SOD-mimetic activity, facilitation of de novo synthesis of Cu2Zn2SOD or MnSOD, or synthesis of other Cu-and Mn-dependent tissue repair enzymes have been suggested as plausible modes of action for Cu(II)2(3,5-DIPS)4 and Mn(3,5-DIPS)6 in accounting for their pharmacological activities [13]. It has since occurred to us that Mn3(3,5-DIPS)6might also down-regulate NADPHdependent enzymes such as PHD and NOS as has been reported for Cu(II)2(3,5-DIPS)4 [10]. To examine this question in a system which is less complicated than NOS but shown to be relevant to this enzyme system [10], the reactivity of Mn3(3,5-DIPS)6 was examined using PHD. These studies revealed that Mn3(3,5-DIPS)6 does down-regulate PHD. This complex as well as Cu(II)2(3,5-DIPS)4 were then examined for their ability to decrease the synthesis of u-Cit by authentic NOS using purified rat brain NOS.
We are reporting the down-regulation of PHD by Mn3(3,5-DIPS)6 whereinthis complex does not inhibit PHD but serves as an electron acceptor in inhibiting reduction of DCPIP, the dye used as the obligate electron acceptor for PHD, and in serving as an electron acceptor this complex may down-regulate rat brain NOS in decreasing the synthesis of -Cit.
Additional support for the suggestion that Cu(II)z(3,5-DIPS)4 may down-regulate NOS is also presented with the demonstration that this complex also decreases the synthesis of I-Citrulline and NO using purified rat brain NOS. It is plausible that the pharmacological activities of Mn3(3,5-DIPS)6 and Cu(II)2(3,5-DIPS)4 may be due in part to down-regulation of NOS.
A 300 mM solution of Tris buffer and 10 mM solution of DCPIP were prepared in deionized Visible and ultraviolet spectrophotometric measurements were performed at 600 nm for DCPIP and 340 nm for NADPH with zero absorbance set with buffer. When Mn3(3,5-DIPS)6 was to be added to a reaction mixture, the studied concentration of Mn3(3,5-DIPS)6, or an equal volume of ethanol, was added to buffer to set zero absorbance and eliminate the absorbance due to the 3,5-DIPS ligand, Lm,x 306 rim.
To determine the effect of Mn3(3,5-DIPS)6 on the PHD enzyme system, a final concentration of 0.0 tM, 3.05 tM, 6.1 laM, or 12.2 laM [200 xL of ethanol or 50 tL, 100 xL, or 200 L of the 200 laM Mn3(3,5-DIPS)6 solution] was added to the reaction mixture. To examine Mn3(3,5-DIPS)6 as an electron acceptor for NADPH reduced PHD, a concentration of 0.0 tM, 2.4 tM, 4.8 laM, 9.5 laM, 19 tM, or 38 laM Mn3(3,5-DIPS)6 was added to the reaction mixture in the absence of DCPIP. The initial rate of 0.37 nmol/min for NADPH oxidation with the addition of 200 lal of ethanol, used to make the 0 tmol addition of Mn3(3,5-DIPS)6 in the absence of DCPlP, was subtracted from initial rates obtained for additions of alcohol solutions of Mn3(3,5-DIPS)6 used to demonstrate recovery of PHD activity. Initial rates were obtained by measuring changes in absorbance at 5 minutes following the addition of enzyme. All determinations were performed in triplicate and results presented as means +S.E.M..
Decreases in absorbance at 340 nm for the oxidation of NADPH and 600 nm for the reduction of DCPIP were measured. It was found that the 340 nm absorbance of DCPIP decreased at the same rate as the rate of decrease of the 600 nm absorbance. For the complete NADPH and DCPIP reaction mixture the 340 nm absorbance is due to the combined absorbances of NADPH and DCPIP at 340 nm. The apparent rate of decrease of the 340 nm absorbance for the complete reaction mixture was found to be approximately twice, 0.224 nmol/min, the rate of decrease of the 600 nm absorbance, 0.099, due to the oxidation of NADPH and reduction of DCPIP. Rat brain NOS was purified from recombinant baculovirus-infected cells as described elsewhere [17]. Synthesis of H--Cit and NO from 3H--Arg was determined by incubation of 0.2 to 0.3 lag of enzyme at 37C for 10 rain in 0.1 ml of a 50 mM triethanolamine hydrochloride buffer (pH 7.0)containing 0.1 mM 3H--Arg (-50,000 cpm), 0.2 mM NADPH, 5 tM flavin adenine dinucleotide, 10 M tetrahydrobiopterin, 0.5 mM CaCI, and 10 lag calmodulin/ml in the presence of Mn3(3,5-DIPS)6 or Cu(II)2(3,5-D!PS)4 (1048 Da for the dihydrate) added as a dimethylsulfoxide solution, followed by isolation of H--Cit by cation exchange chromatography which has been described in detail [18]. All determinations were performed in triplicate except the decrease in -Cit caused by additions of 3tM, 30tM, or 0.5ram Cu(II)2(3,5-DIPS)4 and lmM Mn3(3,5-DIPS)6.
Dimethylsulfoxide solutions of these complexes were used to make additions to the reation mixture. The increase in absorbance at 500 nm was continuously monitored against blank samples containing buffer instead of enzyme, and the amount of enzymatically reduced cytochrome c was calculated using an extinction coefficient of 21 mM cm .D ata are results of a single experiment wherein the control was performed in triplicate and duplicate determinations were performed for the addition of both complexes. Results are presented as means +_S.E.M.

Results
As shown in Figure 1, addition of unit of PHD to a mixture of 114 gM NADPH and 114 gM DCPIP caused the reduction of the obligate electron acceptor DCPIP. The initial rate of decrease in absorbance at 340 nm appeared to be faster than the initial rate of decrease in absorbance at 600 nm due to an absorbance for DCPIP at 340 nm which also decreased at the same rate as the decrease in absorbance at 600 nm with the reduction of DCPIP. Addition of 3 to 12 gM Mn3(3,5-DIPS)6 produced a concentration related decrease in the initial rate of reduction of DCPIP with little change in the initial rate of oxidation of NADPH. The concentration of Mn3(3,5-DIPS)6 required to produce a 50% decrease in reduction of DCPIP by NADPH reduced PHD was 5.0 IaM.
These results demonstrated that PHD oxidation of NADPH occurred in the absence of the reduction of DCPIP, the obligate electron acceptor required for oxidation of NADPH by PHD.
Oxidation of NADPH by PHD in the absence of DCPIP reduction suggested that Mn3(3,5-DIPS)6 served as the obligate electron acceptor in enabling the continued oxidation of NADPH. This suggestion was examined by determining the oxidation of NADPH in the absence of DCPIP and the presence of increasing concentrations of 2.4 to 38 gM Mn(3,5-DIPS)6 to a reaction mixture containing 38 gM NADPH. Recovery of the initial rate of enzymatic oxidation of NADPH was directly related to increasing concentration of added Mn3(3,5-DIPS)6 as shown in Figure  The concentration of Mn3(3,5-DIPS)6 required to produce 50 percent recovery of the maximum initial rate of PHD activity in this system was 6 gM. Under these conditions the role of Mn3(3,5-DIPS)6 appears to be catalytic. This demonstration of enzyme activity recovery is consistent with the suggestion that Mn3(3,5-DIPS)6 does not inhibit PHD in causing the decrease in reduction of DCPIP in the complete NADPH-PHD-DCPIP reaction mixture. The decrease in reduction of DCPIP in the presence of Mn3(3,5-DIPS)6 which enables the oxidation of NADPH supports the suggestion that Mn3(3,5-DIPS)6 does not inhibit PHD but serves to "down-regulate" PHD reduction of DCPIP by serving as an election acceptor for reduced PHD and enabling continued oxidation of NADPH. The decrease in reduction of DCPIP in the presence of Mn3(3,5-DIPS)6 can not be referred to as PHD inhibition and should be viewed as down-regulation of PHD in decreasing the reduction of DCPIP. This interpretation of these results suggested that this complex might also down-regulate NOS.
To examine the possibility that Mn3(3,5-DIPS)6 might down-regulate NOS, purified rat brain NOS was used to measure the conversion of -Arg to -Cit in the presence of Mn3(3,5-DIPS)6. As

Discussion
Continuous oxidation of NADPH by PHD requires the presence of an electron acceptor. 2,6dichlorophenolindophenol serves as an electron acceptor in its role as an in vitro substrate for the determination of PHD activity. The decrease in reduction of DCPIP in the presence of Mn3(3,5-DIPS)6 with continued oxidation of NADPH suggested that Mn3(3,5-DIPS)6 served as an electron acceptor in down-regulating, but not inhibiting, the reduction of DCPIP by NADPH-reduced PHD. Evidence for this down-regulation was obtained when the removal of DCPIP and addition of increasing concentration of Mn3(3,5-DIPS)6 caused a recovery of NADPH oxidation by PHD, demonstrating that PHD activity was not inhibited by Mn3(3,5-DIPS)6.
When recovery was determined for the enzyme system containing 114 gM NADPH and unit of PHD, the addition of up to 48.8 laM Mn3(3,5-DIPS)6 did not yield 100% recovery of the initial rate of NADPH oxidation. However, the system containing 38 gM NADPH and unit of PHD permitted maximum recovery of NADPH oxidation and 50 percent recovery was found for the addition of 6 laM Mn3(3,5-DIPS)6.
It is suggested that Mn3(3,5-DIPS)6 functions in a catalytic role as the electron acceptor in recovery of PHD activity. In the proposed mechanism in which Mn3(3,5-DIPS)6 serves as an electron acceptor in down-regulation of PHD, Mn(III) is reduced to Mn(II) which is in turn oxidized to Mn(III). As shown in Figure 5, oxidation of Mn(II), the product of PHD reduction, to Mn(III) by dissolved oxygen and the disproportionation of the resultant superoxide [12] and/or hydrogen peroxide [16,20]  The net consumption of oxygen in these reactions could also offer an additional accounting for down-regulation of NOS which requires oxygen for the oxidation of L-Arg to L -Cit and NO. Disproportionation of superoxide by Mn3(3,5-DIPS)6 may also appear to reduce superoxide produced by NOS when NOS functions to generate superoxide via a one electron reduction of molecular oxygen [21 ]. Recovery studies also demonstrated that large concentrations of NADPH impede the recovery activity of Mn(3,5-DIPS)6 which was not measurable in the system containing 114 NADPH with up to 48.8 gM Mn3(3,5-DIPS)6. Lowering the concentration of NADPH to 38 allowed the measurement of PHD recovery. The presence of 114gM DCPIP may facilitate downregulation of PHD by Mn3(3,5-DIPS)6 by forming a DCPIP-Mn-3,5-DIPS complex shown in Figure 6. The down-regulation concentration for 50 percent decrease in DCPIP reduction (DRC0) in the 114 gM NADPH and DCPIP system was 5 gM Mn3(3,5-DIPS)6. However, the presence of 114 laM NADPH may inhibit or decrease the effectiveness of Mn3(3,5-DIPS)6 in facilitation recovery of NADPH as a result of the formation of an NADPH complex ( Figure 6). Non-toxic doses of manganese complexes, which would supply less than the daily required intake of manganese, have been shown to have beneficial anticonvulsant effects in models of seizure and radiation injury [11,12]. Very small doses of Mn(3,5-DIPS)6, have both radioprotectant and radiorecovery activities [12]. The plausible down-regulation of NOS by Mn(3,5-DIPS)6may account for its anticonvulsant activity in the Metrazole and Electroshock models of seizure [12]. In addition to Metrazole-induced [22] and electrically kindled seizures [23], seizures induced by N-methyl-D-aspartate (NMDA) receptor activation [24][25][26] Radiation injury is either a local or systemic inflammatory disease depending upon either focal-tissue or whole-body radiation exposure. Since NO has roles in perception of inflammatory insults and mediation of the host response to inflammation [10 and citations therein] and the modulation of NO synthesis is required to bring about cessation of this component of the inflammatory response, the down-regulation of NOS by Mn(3,5-DIPS)6 and Cu(II)z(3,5-DIPS)4 may account for the increase in survival of lethally irradiated mice treated with these complexes [12,13].
It is generally accepted that NOS has features similar to Cytochrome p-450 in incorporating one atom of activated dioxygen into substrate and requiring activation of two molecules of dioxygen in the syntheses of L-Cit and NO from L-Arg [33][34][35]. However, this mechanism has been questioned by Leone et al. [36] who provided evidence consistent with NOS being a dioxygenase wherein both atoms of dioxygen are incorporated into L-Arg to give the two NOS products, NO and L-Cit. While both enzymatic transformations are plausible, the efficiency of a dioxygenase makes this possibility attractive. The isolation of N-hydroxy-L-arginine from a NOS reaction mixture may be a P-450-1ike oxidation product but a kinetically less likely product. In this event, Mn3(3,5-DIPS)6 being an effective SOD-mimetic or serving as an electron acceptor might also downregulate this P-450 activity as has been shown for Cu(II)2(3,5-DIPS)4 and other Cu complexes [37][38][39].
Nitric oxide is a reactive molecule with a half-life of seconds in biological systems [40] and would most likely react with many cellular and extracellular components, causing marked chemical injury before it diffuses to a target cell. Since NO is reactive, NOS may include a component which accepts newly synthesized NO and serves as its transporter. Consequently, suggestions that NO is transported via either an organic thionitrite, nitrosoamine, alkyl and aryl nitrite, or peroxylnitrite or Fe, Mn, or Cu nitrosyl complexes [40,41] merit consideration and further study.
While all of these potential transport forms of NO are plausible, evidence does exist for thionitrites of cysteine and penicillamine [41] in serving as organic NO transporting compounds and a NO complex of Fe, Na2Fe(III)(CN)5(NO), is a well known NO transporting complex used to rescue patients in hypertensive crises. It is also plausible that nanomolar or lower concentrations of a Mn(II) or Cu(II) complex, smaller than the concentration that might down-regulate NOS, can serve as NO transporting complexes, O-N-Mn(III)L3 or O-N-Cu(III)L3 This point may be particularly relevant to the observation that the IC 50 far Mn3(3,5-DIPS)6 in decreasing NOS activity was 0.1 mM supporting the notion that a lower and more physiologically relevant concentration would be more likely to form a NO complex without decreasing NO synthesis. The reported [42] potentiation of the NO-mediated increase in cGMP by a small molecular mass Mn(II) complex (SC52608), while suggested to be due to its SOD-mimetic activity [42], could also be due to its ability to form a O-N-Mn(III) complex and serve as a NO transporting agent. There are many Mn-and Cudependent enzymes and their facitated activation by treatment with a Mn and or Cu complex also offers a plausible accounting for the pharmacological effects of low molecular mass Mn and Cu complexes [13]. Manganeseand Cu-dependent enzymes may also have a biochemical role in mediating normal physiologic roles of NO. Manganese and Cu requirements for normal physiological and biochemical function of NO-mediated events merit further investigation.