Xavier Analúcia Rampazzo, Roselino José Eduardo de Salles, Resano Neusa Maria Zanon, Garófalo Maria Antonieta Rissato, Migliorini Renato Helios, Kettelhut Isis do Carmo
Department of Biochemistry, Immunology and Physiology, School of Medicine of Ribeirão Preto, University of São Paulo, Brazil.
Can J Physiol Pharmacol. 2002 Feb;80(2):164-9. doi: 10.1139/y02-013.
Although the conversion of lactate to glycogen (glyconeogenesis) in muscle was demonstrated a long time ago, the biochemical reactions responsible for this process are still a controversial matter. In the present study, advantage was taken from the specific inhibition induced by phenylalanine on muscle pyruvate kinase (PK) to investigate the role of reverse PK activity in muscle glyconeogenesis. Addition of phenylalanine to the incubation medium of a preparation of isolated, intact skeletal muscles that maintain metabolic activity for several hours reduced by 50% the rate of incorporation of [14C]lactate or [14C]bicarbonate into muscle glycogen. Muscle extracts presented high levels of maximal activity of PK in the reverse direction, which was completely blocked in the presence of phenylalanine. In contrast, mercaptopicolinic acid, an inhibitor of phosphoenolpyruvate carboxykinase (PEPCK), did not affect the incorporation of 14C from either lactate or bicarbonate into muscle glycogen. Maximal PEPCK activity was much lower in muscle extracts than in gluconeogenic or glyceroneogenic tissues and was suppressed in the presence of mercaptopicolinic acid. The data suggest that a reversal of the metabolic flux through the reaction catalyzed by PK contributes to the accumulation of lactate-derived glycogen that occurs in skeletal muscle under certain physiological conditions.
尽管很久以前就已证实肌肉中乳酸可转化为糖原（糖异生），但负责这一过程的生化反应仍是一个有争议的问题。在本研究中，利用苯丙氨酸对肌肉丙酮酸激酶（PK）的特异性抑制作用，来研究逆向PK活性在肌肉糖异生中的作用。向能保持数小时代谢活性的分离完整骨骼肌制剂的孵育培养基中添加苯丙氨酸，可使[14C]乳酸或[14C]碳酸氢盐掺入肌肉糖原的速率降低50%。肌肉提取物中PK的逆向最大活性水平较高，在苯丙氨酸存在下完全被阻断。相比之下，磷酸烯醇式丙酮酸羧激酶（PEPCK）的抑制剂巯基吡啶甲酸并不影响乳酸或碳酸氢盐中14C掺入肌肉糖原。肌肉提取物中的最大PEPCK活性远低于糖异生或甘油异生组织，且在巯基吡啶甲酸存在下受到抑制。数据表明，通过PK催化的反应使代谢通量逆转，有助于在某些生理条件下骨骼肌中由乳酸衍生的糖原积累。
