K P Block

University of Wisconsin, Madison, Madison, MS, United States

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Publications (15)82.15 Total impact

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    K P Block, A E Harper
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    ABSTRACT: Plasma and brain amino acid and plasma branched-chain alpha-keto acid (BCKA) concentrations were measured in rats fed diets containing high levels of individual amino and alpha-keto acids. Consumption of a low-protein (9% casein) diet high in leucine or alpha-ketoisocaproate depressed plasma concentrations of isoleucine and valine and their respective keto acids, alpha-keto-beta-methylvalerate and alpha-ketoisovalerate. High dietary levels of alpha-keto-beta-methylvalerate or alpha-ketoisovalerate (but not of isoleucine or valine) depressed plasma concentrations of the other BCKA and their respective branched-chain amino acids (BCAA). Consumption of a low protein, high phenylalanine diet depressed plasma concentrations of both BCAA and BCKA. Brain large neutral amino acid pools of rats fed all low-protein, high-amino acid diets were depleted. Consumption of diets high in individual BCKA increased brain concentrations of aromatic amino acids. In this study of rats allowed to feed for only 6 h/d, elevated brain phenylalanine concentration was associated with a significant depression of food intake, whereas elevated brain BCAA concentrations were not. Also, elevated plasma BCKA concentrations, comparable with those observed in maple syrup urine disease, were accompanied by elevations in concentrations of aromatic amino acids in brain but not in plasma.
    Journal of Nutrition 06/1991; 121(5):663-71. · 4.20 Impact Factor
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    ABSTRACT: Diets containing high quantities of individual branched-chain alpha-keto acids (BCKAs) or a combination of BCKAs as used for treatment of renal disease were fed to rats. When the diet contained a single BCKA, its concentration was high in plasma and the concentration of its corresponding amino acid was high in plasma and brain. Liver BCKA dehydrogenase (BCKD) was 42% active in control rats. Consumption of diets containing 0.38 mol/kg diet of alpha-ketoisocaproate (KIC), alpha-keto-beta-methylvalerate (KMV), or alpha-ketoisovalerate (KIV) resulted in complete activation of liver BCKD. Consumption of the diet containing the combination of BCKAs increased basal BCKD activity of liver twofold. Muscle BCKD was activated after feeding the KIV diet (2-fold), the KIC diet (3-fold), and the KMV diet (15-fold). Total BCKD activity of liver and muscle was unaffected by dietary treatments. Activation of liver and muscle BCKD by dietary BCKA is consistent with their ability to inhibit BCKD kinase in vitro.
    American Journal of Clinical Nutrition 09/1990; 52(2):313-9. · 6.50 Impact Factor
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    K P Block, R P Aftring, M G Buse
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    ABSTRACT: Effects of feeding frequency on liver branched-chain alpha-keto acid dehydrogenase (BCKAD) activity are unknown. In the present study, rats were trained to consume their daily allotment of food in 6 h (meal-feeding). Rats were fed diets containing 0, 9, 25 or 50% casein and after 10 d were killed before or 3 h after the meal. The enzyme in rats fed diets containing 0, 9 and 25% casein was activated three- to sixfold after meal consumption. Previous studies showed that the liver enzyme is essentially fully activated in post-absorptive rats fed an adequate protein diet ad libitum. Meal-feeding an adequate protein (25% casein) diet resulted in a marked decrease in the postabsorptive percentage of active complex compared to ad libitum feeding of the same diet (29 +/- 6% vs. 93 +/- 6% active). Administration of alpha-ketoisocaproate (200 mumol/100 g body weight, an inhibitor of BCKAD kinase) reversed the meal-feeding-induced inactivation of the complex within 10 min. We conclude that the frequency of food intake, in addition to the level of dietary protein, influences the proportion of liver BCKAD in the active state. Inactivation of hepatic BCKAD in rats trained to feed once a day may be an adaptive mechanism that results in increased efficiency of branched-chain amino acid utilization between meals.
    Journal of Nutrition 08/1990; 120(7):793-9. · 4.20 Impact Factor
  • K P Block, M G Buse
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    ABSTRACT: Branched-chain alpha-keto acid dehydrogenase (BCKAD) is a multisubunit complex regulated by phosphorylation and is considered to be rate-limiting for branched-chain amino acid (BCAA) metabolism in skeletal muscle. Glucocorticoids increase net protein degradation in muscle; associated with this increased breakdown of muscle protein is an elevated rate of BCAA oxidation. The effects of glucocorticoids on skeletal muscle BCKAD were investigated in different rat models. BCKAD was activated after glucocorticoid treatment (both acutely, within 2 h, and chronically). The amount of enzyme per muscle cell increased after 5 d of cortisone acetate treatment. Insulin administration partially blocked the acute effects of glucocorticoids on muscle BCKAD. Activation was also observed during metabolic acidosis, insulinopenic diabetes mellitus, and endotoxic shock, three conditions characterized by elevated circulating glucocorticoids, increased BCAA oxidation, and increased net protein breakdown. Activation of BCKAD may account for the increased oxidation of BCAA observed during hypercortisolemia. The sequelae of this accelerated catabolism may include increased glutamine and alanine production for gluconeogenesis and provision of ATP for muscle work.
    Medicine &amp Science in Sports &amp Exercise 07/1990; 22(3):316-24. · 4.48 Impact Factor
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    ABSTRACT: Protein catabolic states (i.e., sepsis and trauma) are thought to be associated with accelerated oxidation of branched-chain amino acids (BCAA). Branched-chain alpha-keto acid dehydrogenase (BCKAD), the rate-limiting enzyme for BCAA oxidation by muscle, is regulated by phosphorylation/dephosphorylation. Skeletal muscle BCKAD was only 2-4% active in control rats. Intravenous injection of Salmonella enteritidis endotoxin (0.25-10 mg/kg) did not change total BCKAD activity, but increased the percent active enzyme in muscle three- to four-fold in 4-6 h. Identical results were observed in adrenalectomized rats pretreated with one dose of alpha-methylprednisolone (2.5 mg/kg i.p.) 30-60 min before saline or endotoxin injection, indicating that endotoxin's effect was not mediated by hypersecretion of adrenal hormones. Cortisone pretreatment of normal rats (100 mg/kg per d) for 2 d prevented endotoxin-induced activation of muscle BCKAD, suggesting that endogenous secretion products mediated BCKAD activation by endotoxin. Human recombinant tumor necrosis factor-alpha and/or IL-1 beta or alpha (50 micrograms/kg) increased muscle BCKAD activation two- to fourfold in normal rats 4-6 h after intravenous injection. We conclude that cytokine-mediated activation of muscle BCKAD may contribute to accelerated BCAA oxidation in septicemia.
    Journal of Clinical Investigation 02/1990; 85(1):256-63. · 12.81 Impact Factor
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    ABSTRACT: The relationships among dietary protein intake, plasma branched-chain amino acid (BCAA) and keto acid (BCKA) concentrations, and liver BCAA-degrading enzyme activities were investigated in rats fed, for 5 h/d for 2, 6 or 9 d, diets containing from 0 to 50% casein. Plasma, liver and muscle BCAA concentrations were proportional to protein intake over the entire range tested; plasma BCKA concentration, however, was proportional only in the range from 0 to 20% casein, after which a plateau was reached. By d 2, liver cytosolic BCAA aminotransferase activity had increased in rats fed 50% casein; by d 9, activity had increased in rats fed 0 or 5% casein as well. Liver mitochondrial BCAA aminotransferase activity was unresponsive to dietary treatment. Basal liver BCKA dehydrogenase activity and the percent active complex were proportional to protein intake on d 2 and 6. On d 2, total BCKA dehydrogenase activity was the same in all groups; by d 6, total activity had increased in rats fed 30 or 50% casein. We conclude that although the adaptive changes in BCAA-degrading enzyme activities are small, they are sufficient to compensate for excessively high or low protein intakes, so that tolerable concentrations of BCAA and BCKA are maintained.
    Journal of Nutrition 09/1989; 119(8):1203-12. · 4.20 Impact Factor
  • Methods in Enzymology 02/1988; 166:201-13. · 2.00 Impact Factor
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    ABSTRACT: Branched-chain amino acid (BCAA) metabolism is frequently abnormal in pathological conditions accompanied by chronic metabolic acidosis. To study how metabolic acidosis affects BCAA metabolism in muscle, rats were gavage fed a 14% protein diet with or without 4 mmol NH4Cl X 100 g body wt-1 X day-1. Epitrochlearis muscles were incubated with L-[1-14C]-valine and L-[1-14C]leucine, and rates of decarboxylation, net transamination, and incorporation into muscle protein were measured. Plasma and muscle BCAA levels were lower (P less than 0.05) in acidotic rats. Rates of valine and leucine decarboxylation and net transamination were higher (P less than 0.05) in muscles from acidotic rats; these differences were associated with a 79% increase in the total activity of branched-chain alpha-keto acid dehydrogenase and a 146% increase in the activated form of the enzyme. We conclude that acidosis affects the regulation of BCAA metabolism by enhancing flux through the transaminase and by directly stimulating oxidative catabolism through activation of branched-chain alpha-keto acid dehydrogenase.
    The American journal of physiology 07/1987; 252(6 Pt 1):E712-8. · 3.28 Impact Factor
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    ABSTRACT: The effects of dietary protein on the activity of skeletal muscle branched-chain alpha-keto acid dehydrogenase (BCKAD) were investigated. BCKAD is rate-limiting for branched-chain amino acid (BCAA) catabolism by muscle; its activity is modulated by phosphorylation-dephosphorylation. In rats fed an adequate protein (25% casein) diet, BCKAD was approximately 2% active postabsorptively and increased to 10% or 16% active after a 25% or 50% protein meal, respectively. Prolonged feeding of a 50% protein diet increased postabsorptive BCKAD activity to 7% with further increases to 40% active postprandially. On a low protein (9% casein) diet BCKAD remained approximately 2% active regardless of meal-feeding. Dose-dependent activation of BCKAD by intravenous leucine in postabsorptive rats was blunted by a low protein diet. We conclude that excesses of dietary protein enhance the capacity of skeletal muscle to oxidize BCAA, muscle conserves BCAA when protein intake is inadequate, and skeletal muscle may play an important role in whole-body BCAA homeostasis.
    Journal of Clinical Investigation 06/1987; 79(5):1349-58. · 12.81 Impact Factor
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    ABSTRACT: Muscle branched-chain alpha-keto acid dehydrogenase, the rate-limiting enzyme for branched-chain amino acid oxidation in skeletal muscle, was measured after treatment of rats with glucocorticoids. Cortisone treatment (10 mg X 100 g body wt-1 X day-1 for 2-5 days) resulted in an approximate doubling of the percentage of active enzyme. To further characterize this effect, the enzyme complex was measured 4 h after the intraperitoneal injection of 6 alpha-methylprednisolone, a water-soluble glucocorticoid with rapid onset effects. The percentage of active enzyme increased linearly as the dose of methylprednisolone was increased from 0.125 to 12.5 mg/100 g body wt, while total enzyme activity was unchanged. Administration of insulin with glucose had no significant effect on the activity of the enzyme. However, treatment of rats with insulin and glucose after methylprednisolone administration partially blocked branched-chain alpha-keto acid dehydrogenase activation. The activity of the enzyme complex was correlated with the concentration of leucine in plasma and muscle. Activation of skeletal muscle branched-chain alpha-keto acid dehydrogenase by increased glucocorticoids may play a role in the acceleration of branched-chain amino acid oxidation observed during severe stress.
    The American journal of physiology 04/1987; 252(3 Pt 1):E396-407. · 3.28 Impact Factor
  • R P Aftring, K P Block, M G Buse
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    ABSTRACT: The response of rat skeletal muscle branched-chain alpha-keto acid dehydrogenase to administration of branched-chain amino acids in vivo was determined using a soluble preparation of the enzyme. After detergent extraction of the complex in the presence of kinase and phosphatase inhibitors, initial in vivo activity was typically 1 nmol X min-1 X g muscle-1, with 0.1 mM alpha-[1-14C]ketoisocaproate as substrate. Total activity of the dephosphorylated complex, measured after preincubation with 15 mM Mg2+, typically reached a maximum of 29 nmol X min-1 X g-1. Thus in overnight-fasted rats the complex was 2-3% active. Initial activity increased three- to fivefold after leucine or isoleucine (at higher concentrations) but not valine administration in vivo. After intravenous leucine injection (0.25 mmol/kg) initial muscle enzyme activity increased rapidly and subsequently decreased, paralleling plasma leucine concentrations, while plasma valine and isoleucine decreased. In conclusion, muscle branched-chain alpha-keto acid dehydrogenase complex is rapidly activated when circulating leucine is increased to concentrations that may occur after meals. During hyperleucinemia accelerated valine and isoleucine degradation by muscle may account for the observed "antagonism" among the branched-chain amino acids.
    The American journal of physiology 06/1986; 250(5 Pt 1):E599-604. · 3.28 Impact Factor
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    ABSTRACT: The activity of liver branched-chain 2-oxo acid dehydrogenase complex was measured in rats fed on low-protein diets and given adrenaline, glucagon, insulin or dibutyryl cyclic AMP in vivo. Administration of glucagon or adrenaline (200 micrograms/100 g body wt.) resulted in a 4-fold increase in the percentage of active complex. As with glucagon and adrenaline, treatment of rats with cyclic AMP (5 mg/100 g body wt.) resulted in marked activation of branched-chain 2-oxo acid dehydrogenase. Insulin administration (1 unit/100 g body wt.) also resulted in activation of enzyme; however, these effects were less than those observed with glucagon and adrenaline. In contrast with the results obtained with low-protein-fed rats, administration of adrenaline (200 micrograms/100 g body wt.) to rats fed with an adequate amount of protein resulted in only a modest (14%) increase in the activity of the complex. The extent to which these hormones activate branched-chain 2-oxo acid dehydrogenase appears to be correlated with their ability to stimulate amino acid uptake into liver.
    Biochemical Journal 01/1986; 232(2):593-7. · 4.65 Impact Factor
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    ABSTRACT: As part of an effort to explain the leucine-induced depressions of plasma isoleucine and valine concentrations, and the concomitant stimulation of valine oxidation in vivo, branched-chain alpha-keto acid dehydrogenase (BCKAD) activity was measured in livers from rats that were fed for only 6 h/d large quantities of individual amino acids in a low protein diet. Preincubation of homogenates with buffer containing Mg2+ and Ca2+ allowed estimation of fully active complex. Cytosolic and mitochondrial branched-chain-amino-acid aminotransferase (BCAAT) activities were also measured in livers of rats fed an excess of leucine. The percentage of BCKAD in the active form in livers of rats fed the low protein diet containing an excess of leucine, isoleucine, valine or phenylalanine for 2 d was double that of rats fed the low protein control diet (control, leucine, isoleucine, valine and phenylalanine groups having, respectively, 45 +/- 2, 85 +/- 7, 85 +/- 3, 95 +/- 5, and 81 +/- 4% of hepatic BCKAD in the active form). Consumption of a low protein diet containing an excess of leucine had no significant effect on either cytosolic or mitochondrial BCAAT activities of liver. The response of BCKAD in liver can contribute to the leucine-induced stimulation of valine oxidation in vivo but analysis of the results of this study leads to the conclusion that other mechanisms, probably in nonhepatic tissues, must also be involved.
    Journal of Nutrition 01/1986; 115(12):1550-61. · 4.20 Impact Factor
  • K P Block, A E Harper
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    ABSTRACT: The short-term effects of feeding rats high levels of L-leucine or L-isoleucine on valine metabolism in vivo have been investigated. Consumption of a low-protein diet containing an additional 5% of leucine resulted in depression within one hour of the plasma concentrations of isoleucine, valine, alpha-keto-beta-methylvalerate, and alpha-ketoisovalerate. Concurrently with these changes in blood branched-chain amino acids and branched-chain ketoacids was a rapid increase (51%) in whole-body L-[1-14C]-valine oxidation. Studies with intragastrically administered leucine solutions indicated that the depressions in blood concentrations of valine occurred over the same time period as the stimulation in valine oxidation. In contrast, consumption of a low-protein diet containing an additional 5% of isoleucine had no significant effect on the plasma concentrations of leucine, valine, and alpha-ketoisocaproate; a significant (P less than 0.01) depression in the plasma concentration of alpha-ketoisovalerate was observed three hours after the diet containing excess isoleucine had been consumed. In contrast to the results obtained with excess leucine, consumption of excess isoleucine had no significant effect on the rate of valine oxidation in vivo. As part of an effort to explain the leucine-induced depletion of plasma valine and stimulation of valine oxidation, liver and muscle branched-chain aminotransferase and liver branched-chain ketoacid dehydrogenase activities were measured. Consumption of excess leucine had no significant effect on either muscle or liver aminotransferase activities, but was associated with a greater than two-fold increase in hepatic dehydrogenase activity.(ABSTRACT TRUNCATED AT 250 WORDS)
    Metabolism 07/1984; 33(6):559-66. · 3.10 Impact Factor
  • A E Harper, R H Miller, K P Block
    Annual Review of Nutrition 02/1984; 4:409-54. · 9.16 Impact Factor

Publication Stats

508 Citations
82.15 Total Impact Points


  • 1984–1991
    • University of Wisconsin, Madison
      • • Department of Biochemistry
      • • Department of Nutritional Sciences
      Madison, MS, United States
  • 1990
    • Medical University of South Carolina
      • Department of Medicine
      Charleston, SC, United States