Mechanism of action of salsolinol on tyrosine hydroxylase

Tyrosine hydroxylase (TH) is the first and rate-limiting enzyme in dopamine synthesis. Dopamine regulates TH as an end-product inhibitor through its binding to a high and low affinity site, the former being abolished by Ser40 phosphorylation only, and the latter able to bind and dissociate according to intracellular dopamine levels. Here, we have investigated TH inhibition by a dopamine metabolite found in dopaminergic brain regions, salsolinol (SAL). SAL is known to decrease dopamine in the nigrostriatal pathway and mediobasal hypothalamus, and to also decrease plasma catecholamines in rat stress models, however a target and mechanism for the effects of SAL have not been found. We found that SAL inhibits TH activity in the nanomolar range in vitro, by binding to both the high and low affinity dopamine binding sites. SAL produces the same level of inhibition as dopamine when TH is non-phosphorylated. However, it produces 3.7-fold greater inhibition of Ser40-phosphorylated TH compared to dopamine by competing more strongly with tetrahydrobiopterin, the cofactor of this enzymatic reaction. SAL’s potent inhibition of phosphorylated TH would prevent TH from being fully activated to synthesise dopamine.     

The cortical layer of the cytoplasm--a possible site of the action of prenervous transmitters

Specific and opposite effects of some transmitter antagonists (beta-adrenoblockers, serotonin antagonists) on mechanical properties of the surface of fertilized eggs of the sea urchin Paracentrotus lividus have been demonstrated. Possible role of endogenous transmitters in regulation of the properties of cytoplasmic cortex is discussed.     

The cytoplasmic membrane as the site of the antimicrobial action of N-octylethanolamine

On agar spread plates, N-octylethanolamine was biocidal at comparable minimum inhibitory concentration (MIC) values (3–4mm) against Pseudomonas aeruginosa (two strains), Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Candida tropicalis, and Acremonium sp. which had been grown on a number of different media. The inhibition was greater at higher pH values. In liquid culture, growth inhibition by 3mm N-octylethanolamine was accompanied by cell lysis. Both effects could be prevented by the presence of 1mm spermine or spermidine, but only in bacteria, and not at high pH values. These effects of the polyamines were shown to be non-specific, being shared by other polycations and Mg²⁺ ions. N-Octylethanolamine at concentrations above its MIC caused total inhibition of the oxidation of 1mm glucose by Ps. aeruginosa (CAS1 and PAO1), E. coli, or C. tropic an effect that was partially reversible by Mg²⁺ ions. At concentrations below the MIC, there was little inhibit ion of glucose oxidation but a potent inhibition of the extrusion of ethidium bromide from intact cells of E. coli, suggesting that at such concentrations N-octylethanolamine is uncoupling oxidative phosphorylation. The data presented confirm the view that the biocidal effects are due to action on the cytoplasmic membrane.     

A possible site of action for adipokinetic hormone on the flight muscle of locusts

In mitochondrial preparations the oxidation of palmitate and of palmitoyl carnitine is stimulated by dilute extracts of the glandular lobes of the corpora cardiaca. Extracts of the storage lobes of the corpora cardiaca or of corpora allata are without effect. In a working single muscle preparation the entry of diglyceride into the muscle is also stimulated by tissue extracts containing adipokinetic hormone. This stimulation is, however, prevented by the addition of 2-bromostearic acid to the perfusion fluid. It is suggested that adipokinetic hormone may have a single site of action in the flight muscle and this may be in stimulating the entry of acyl groups into the mitochondria; perhaps by acting on the inner acyl carnitine transferase.     

Presence and possible site of action of secretin in the rat pituitary and hypothalamus

Secretin-like immunoreactivity was detected in extracts of several rat brain structures by radioimmunoassay, most notably in the pituitary, hypothalamus, pineal and septum. Its localization to these structures suggested that it might play a role in neuroendocrine events similar to its structural homolog vasoactive intestinal peptide. Dose-related stimulations (MED, 10(-7) M) of prolactin (PRL) release were observed after incubation of synthetic secretin with dispersed, cultured pituitary cells from male and ovariectomized (OVX) female rats. In OVX females, i.v. infusion of a high dose of secretin (10 micrograms) resulted in a significant elevation of PRL levels. Doses of secretin as low as 0.1 micrograms when administered into the third cerebroventricle were capable of significantly inhibiting PRL release in both males and OVX females, suggesting an ultrashort-loop, negative feedback of secretin. Secretin can now be added to the growing list of putative PRL-releasing agents.     

The leaf as the site of gibberellin action in flower formation in Bryophyllum daigremontianum

Summary The long-short-day plant Bryophyllum daigremontianum can be induced to flower by transfer from long to short days (LDSD), or by gibberellin (GA) application under SD. Application of GA to mature leaves of intact or partially defoliated plants induces flowering more effectively than when applications are made to the youngest leaf pair and the shoot tip.Mature leaves on de-budded plants in SD are induced to produce floral stimulus by GA application, as demonstrated by grafting LD receptor scions onto the debudded plants, or by grafting SD leaves treated with GA onto receptor stocks in LD. This shows that GA applied to Bryophyllum in SD exerts its flower-promoting effect in the leaves.The minimal number of SD necessary for flower formation in Bryophyllum is approximately 15, both in case of photoinduction by the shift LDSD, and after GA treatment in SD. It is concluded that the LD part of photinduction establishes a high level of endogenous GAs in the leaves which is a prerequisite for production of floral stimulus under subsequent SD.Work supported by the United States Atomic Energy Commission, Contract No. AT(11-1) 1338.     

GTP-binding proteins as possible targets for protein kinase C action

The alpha-subunits of two guanine nucleotide binding proteins Gi and transducin, as well as the beta-subunit of transducin, serve as substrates for phosphorylation by the Ca2+- and phospholipid-dependent protein kinase C (PKC). Phosphorylation of the alpha-subunit of transducin is strictly dependent on its conformation and it is only the inactive form that is subjected to phosphorylation by PKC. This review will focus on the proposition that G proteins may serve as cellular targets for modulatory actions of PKC.     

Liver aldolase as a possible target for the action of N-methyl-N-nitrosourea

The effect of N-methyl-N-nitrosourea (MNU) on the activity of cytoplasmic and reversibly bound to subcellular structures liver aldolase was studied. In vitro, the activity of aldolase purified from rabbit muscles is inhibited by MNU by 70-80% relative to fructose-1,6-diphosphate and by 50-60% relative to fructose-1-phosphate. These substrates and the competitive inhibitor ATP do not protect the enzyme against the inactivation by MNU. MNU inhibits the activity of cytoplasmic aldolase by 30-40% and 20% 2-24 hours after a single injection (80 mg/kg) in vivo. The enzyme affinity for fructose-1,6-diphosphate is markedly decreased (2-fold). Activation of cytoplasmic aldolase relative to both substrates, which is especially well-pronounced with fructose-1-phosphate after inhibition of the enzyme activity, was observed. The enzyme activity relative to both substrates was found to increase in the mitochondrial and nuclear fractions within 48 hours. MNU has no effect on the activity of aldolase bound to microsomes. MNU influences the aldolase binding to organelle membranes. MNU injections at early periods (2-168 hours) accounts for the differences in the kinetic properties of cytoplasmic and reversibly bound to subcellular structures liver aldolase. These changes persist within 168 hours after MNU administration and may result in disturbances in cell metabolism as well as in the regulation of metabolic pathways, such as glycolysis and gluconeogenesis.     

The possibility of peripheral cholinolytic action of psychostimulants

The cholinolytic effect of sydnophen discovered in earlier anesthetized cats was confirmed on unanesthetized fish and frogs: the vagal bradycardia induced by electric stimulation of peripheral vagal end was decreased or even abolished by intravenous injection of sydnophen (0.2-20 mg/kg). The amphetamine (0.2-30 mg/kg) also blocked the vagal bradycardia in anesthetized cats and unanesthetized frogs. The maximum vagolytic action of amphetamine appeared later (in 4-8 min after injection) in compared with sydnophen (1-3 min). The small dose of amphetamine (0.2-0.3 mg/kg) in contrast to sydnophen didn't decrease the vagal bradycardia but even increased it. It was suggested that the cholinolytic effect of sydnophen and amphetamine is due to different mechanisms.     

Plastocyanin as the possible site of photosynthetic electron transport inhibition by glutaraldehyde

Treatment of spinach chloroplasts with glutaraldehyde causes an inhibition in the electron transport chain between the two photosystems. Measurements of O₂ flash yields, pH exchange, and fluorescence induction show that the O₂ evolving apparatus, photosystem II and its electron acceptor pool are not affected. The behavior of P700 indicates that its reduction but not its oxidation, is severely inhibited. Cytochrome f is still reducible by photosystem II but also slowly oxidizable by photosystem I. The sensitivity of isolated plastocyanin to glutaraldehyde further supports the conclusion that glutaraldehyde inhibits at the plastocyanin level and thereby induces a break between P700 and cytochrome f.     

The site of action of the F transfer inhibitor

Summary The mechanism of inhibition of F transfer from E. coli K12 cells containing the fin ⁺ R factor R100 was studied. For this, a series of Flac double mutants carrying both a traO ^-mutation, which prevents function of the transfer inhibitor, and a suppressible mutation in one of ten genes required for conjugational DNA transfer, were constructed. The levels of retransfer of these elements from cells carrying a wild-type Fhis element and R100 showed that of the ten transfer genes, only traJ was directly affected by the transfer inhibitor. Furthermore, in the case of R100 and therefore probably in the case of F itself, it was shown that the products of the other nine genes are absent from the cell during transfer inhibition, suggesting that the traJ product is required for their synthesis.D.F. acknowledges the support of a George Murray Scholarship from the University of Adelaide, Australia.