Stimulus-secretion coupling of arginine-induced insulin release. Functional response of islets to L-arginine and L-ornithine.

L-Arginine and L-ornithine stimulate insulin release from pancreatic islets exposed to D-glucose. This coincides with an increased outflow of 86Rb and 45Ca from prelabelled islets and an increased net uptake of 45Ca by the islets. In the presence of D-glucose, L-lysine stimulates insulin secretion to the same extent as L-arginine or L-ornithine, but the hormonal release is not further enhanced by combinations of these cationic amino acids. L-Arginine or L-ornithine failed to enhance insulin release evoked by either L-leucine or 2-ketoisocaproate. The inhibitor of ornithine decarboxylase D,L-alpha-difluoromethyl ornithine failed to affect the metabolism and insulinotropic action of D-glucose in pancreatic islets, and only caused a partial inhibition of the secretory response to either L-arginine or L-ornithine. The latter amino acids inhibited modestly but significantly D-glucose utilization and oxidation by pancreatic islets. These and complementary findings suggest that the secretory response to L-arginine and L-ornithine is not attributable to any major change in the overall oxidative catabolism of nutrients, but involves mainly a biophysical component, such as the depolarization of the plasma membrane by these cationic amino acids.

Regulation of polyamine synthesis by dietary alpha-aminoisobutyric acid and ornithine.

An experiment was conducted to determine the effect of feeding ornithine in combination with alpha-aminoisobutyric acid (AIB), an inhibitor of arginase, on the regulation of polyamine synthesis in chicks. A total of 48 chicks with genetically elevated renal arginase activity was fed diets containing crystalline amino acids and 1% AIB with or without 2% ornithine. Feeding AIB reduced renal arginase activity, while renal and hepatic ornithine decarboxylase (ODC) activity increased. Feeding AIB plus ornithine caused no further reduction in renal arginase activity compared with that in chicks fed the AIB-supplemented diet. Renal and hepatic ODC activities, however, fell to below control levels. Renal, hepatic, and breast muscle ornithine concentrations increased substantially when ornithine was fed. AIB plus ornithine increased renal putrescine and spermidine concentrations. It was concluded that AIB could partially overcome the ornithine-induced inhibition of ODC activity. These findings support the hypothesis that dietary manipulation of precursor amino acids of polyamines in the presence of metabolites that induce ODC activity can influence tissue polyamine concentrations.

Prevention of a plant disease by specific inhibition of fungal polyamine biosynthesis.

DL-alpha-Difluoromethylornithine (DFMO), an inhibitor of the polyamine biosynthetic enzyme ornithine decarboxylase (EC 4.1.1.17), strongly retards the growth of several species of phytopathogenic fungi in vitro. Such inhibition can be completely reversed by putrescine or spermidine, confirming the essentiality of polyamines for growth of fungal hyphae. We now show that DFMO can protect bean plants (Phaseolus vulgaris Linnaeus cv. Pinto) against infection by uredospores of the bean rust fungus, Uromyces phaseoli Linnaeus, race O. Unifoliolate leaves of 10-day-old greenhouse-grown seedlings were sprayed with 400 microliter per leaf of DFMO at various concentrations in 0.01% Tween 20 at pH 7.0 before or after inoculation with uredospores of Uromyces. After 16 hr in darkness in dew chambers to facilitate spore germination, plants were transferred to the greenhouse, arranged randomly, and examined for local lesions 7 days later. All concentrations of DFMO 0.50 mM or higher gave complete protection against the pathogen; at lower concentrations, postinoculation treatments with DFMO were generally more effective than preinoculation. The appearance of lesions on plants treated with lower concentrations of DFMO was retarded 2-6 days. DFMO also confers protection on unsprayed parts of treated plants, indicating the translocation of some protective effect from sprayed areas. DL-alpha-Difluoromethylarginine, an analogous inhibitor of arginine decarboxylase (EC 4.1.1.19), which is the rate-limiting enzyme in an alternative pathway for polyamine biosynthesis in higher plants, confers no protection even at 5 mM. This emphasizes ornithine decarboxylase as the biochemical locus of choice for the prevention of plant diseases by inhibiting polyamine metabolism.

New therapeutic approaches for brain tumors.

In the last decade, advances in the treatment of primary neuroglial tumors of the central nervous system in adults have been modest. Theories regarding their resistance to treatment have changed little, although now the heterogeneity of anaplastic astrocytomas is recognized. The most effective chemotherapeutic agents--the nitrosoureas and procarbazine--have been used for more than a decade, with no comparably active drugs identified in the meantime. The authors have initiated clinical trials using inhibitors of polyamine synthesis and augmentation of the known effectiveness of irradiation. These programs will be described with preliminary observations.

The influence of cysteine and methionine supplements on polyamine biosynthesis in the rat.

Previous reports from this laboratory have shown that supplementation of diets containing the optimal level (0.02%) of dietary inorganic sulfate (SO4(2-] with cysteine instead of methionine can affect several metabolic pathways. It is possible that these results reflect alterations in the biosynthesis of potent physiological compounds, the polyamines. Adult male albino rats were fed diets containing 15% casein and a constant level of inorganic sulfate (0.02%) supplemented with cysteine (0.505%) or methionine (0.62%). The polyamines (putrescine, spermidine and spermine) and the controlling enzymes for their biosynthesis ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMD) were evaluated in liver, kidney and brain tissue homogenates following a 17-day dietary period. Rats fed the diet supplemented with cysteine had increased ODC activity and decreased SAMD activity when compared to rats fed diets supplemented with methionine. Polyamine concentrations varied in tissues with a trend toward increasing amounts in animals fed the cysteine-supplemented diet. Based on these data, it appears that dietary cysteine stimulates the biosynthesis and increased tissue concentration of polyamines.

Polyamines in the synthesis of bacteriophage deoxyribonucleic acid. II. Requirement for polyamines in T4 infection of a polyamine auxotroph.

Polyamine depletion produced by exogenous arginine in Escherichia coliK-12 cultures defective in agmatine ureohydrolase activity resulted in a marked inhibition of the rates of growth and nucleic acid synthesis. Addition of putrescine or spermidine to such depleted cultures restored the control rate of growth and nucleic acid accumulation. The omission of lysine resulted in a further decrease in the rates of growth and nucleic acid synthesis in polyamine-depleted cells. The addition of exogenous cadaverine increased the rates of growth and ribonucleic acid synthesis to those observed in lysine-supplemented cultures, suggesting that lysine or a derivative of lysine serves a function similar to cadaverine. Addition of lysine to polyamine-depleted cultures at neutral pH results in the synthesis of cadaverine and a new spermidine analogue, both containing lysine carbon. This new metabolite has been isolated and identified as N-3-aminopropyl-1, 5-diaminopentane. T4D infection of the polyamine-depleted mutant resulted in a very low rate of DNA synthesis and phage maturation. The addition of putrescine or spermidine 15 min before infection restored phage DNA synthesis and phage maturation to control rates, i.e., rates observed in infected cells grown in the absence of arginine.