Inosinate dephosphorylation

The enzyme 5 -nucleotidase dephosphorylates IMP to inosine and P. Thus, since this reaction represents a possible fate for the IMP formed by the transferase (Fig. 10.7), reconstitution studies were undertaken with the nucleotidase. These studies were carried out using the HPLC assay method developed for the HGPRTase activity. A reaction mixture was prepared that contained hypoxanthine and PRibPP as substrates. The reaction was started by the addition of purified HGPRTase enzyme. Samples were removed and were analyzed by HPLC. The chromatographic profiles obtained at 0,10, 20, and... 

Gulland and Jackson performed some experiments with 5-nucleotidase, a highly specific enzyme which dephosphorylates 5-phospho-adenosine and -inosine but not" 5-phospho-guanosine and -uridine it is apparently not yet known whether the enzyme dephosphorylates 5-phos-pho-cytidine. They found that a mixture of phosphodiesterase with 5-nucleotidase liberates 35% of the total phosphorus as inorganic phosphate, and therefore decided that two or more of the phosphoryl groups may be attached at position (5) of the ribose units. The 35% dephosphorylation, intermediate between 25 and 50%, was explained as the result of simultaneous, competitive diesterase action at A and B, on two or more phosphoryl groups ... 

Dephosphorylation of inosine monophosphate (IMP) in purine catabolism yields inosine. 

At very low values of EC, when AMP is elevated it is deaminated via AMP deaminase to inosine monophosphate (IMP). This further displaces the adenylate kinase reaction in the direction of ATP synthesis. The IMP is dephosphorylated by nucleotide phosphatase, and the inosine is phosphorylyzed via purine nucleotide phosphorylase, releasing hypoxanthine and ribose 1-phosphate. The latter is metabolized via the pentose phosphate pathway, and most of the carbon atoms enter glycolysis. Because this course of events depletes the overall adenine nucleotide pool, and hence the scope for ATP production in the longer term, it represents a metabolic last ditch stand by the cell to extract energy even from the energy currency itself ... 

Inosine, /nov kypoxamtkmosiae, hypoxanthlae riboside, 9P-D-ribofimmo lkypoxaiilkme a P-glycosi-dic nucleoside of n-ribose and hypoxanthine, M, 268.23, m.p. 218°C (d.), [a]g -73.6° (c = 2.5, 0.01 M NaOH). It occurs free, is particularly abundant in meat and yeast, and is formed by dephosphorylation of inosine phosphates. It fulfills a sprafic function as a component of the anticodon of certain tRNA spe-des (see Rare nncleic add bases). 

Although several enzymes can in theory convert purine ribonucleotides to ribonucleosides, in only a few cases is it known exactly which is acting. Certain strains of Bacillus svbtilis accumulate nucleotides extra-cellularly, but mutants lacking 5 -nucleotidase or alkaline phosphatase, or both, accumulate smaller quantities of nucleotides than cells which have both enzymes. 5 -Nucleotidase appeared to be quantitatively the more important for nucleotide dephosphorylation in these cells 23). Baer and Drummond 12) compared the rates of dephosphorylation of the 2 -, 3 -, and 5 -phosphates of adenosine by perfused rat heart and concluded that only a specific 5 -nucleotidase was actively in contact with the blood. More evidence for the action of specific, rather than nonspecific phosphatases, are the observations that in one system or another, adenylate, inosinate, xanthylate, or guanylate appears to be the nucleotide most rapidly de-phosphorylated. 

In rat heart the control rate of adenylate deaminase activity was lower, and that of adenylate dephosphorylation higher than in lung. Most of the ammonia formed from adenylate was therefore due to adenosine deaminase activity. ATP stimulated adenylate deaminase to almost the same relative degree in heart as in lung, but due to a marked inhibition of dephosphorylation the total amoimt of ammonia formed was less in heart. These data also raise questions concerning the identity and substrate specificities of the enzyme(s) that dephosphorylate adenylate and inosinate. 

The main sources of ribose in meat are inosine 5 -monophosphate (IMP) and smaller quantities of ribose 5-phosphate and free ribose. The IMP is formed in muscle post slaughter from the enzymatic dephosphorylation and deamination of adenosine triphosphate (ATP), the ribonucleotide that is essential to muscle function in the live animal [5]. Further enzymatic breakdown of IMP may lead to hypoxanthine, ribose, and ribose 5-phosphate (Fig. 1), although most of the ribose in meat remains bound within IMP. 

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