. Cell chemistry; a collection of papers dedicated to Otto Warburg on the occasion of his 70th birthday. Warburg, Otto Heinrich, 1883-; Biochemistry. 3o8 F. LYNEN, S. OCHOA VOL. 12 (1953) non-labelled acetoacetyl-S-CoA from the last four carbons of the fatty acid chain reacts with thiolase to give non-labelled acetyl-S-enzyme and this in turn reacts with labelled O acetyl-S-CoA from the pool to yield CH3—CO—CH,—C*—S—CoA. This is shown schemat- ically in Fig. 8 for the case of caproic acid. O CH,—CH,—CH.,—CO- O CHj—'CHo—CHg—'C- O -CHj—C —S—CoA (HS—Enz) O -S—Enz CH3—C —S—CoA (HS—CoA) CH3—CH,—CH.

. Cell chemistry; a collection of papers dedicated to Otto Warburg on the occasion of his 70th birthday. Warburg, Otto Heinrich, 1883-; Biochemistry. 3o8 F. LYNEN, S. OCHOA VOL. 12 (1953) non-labelled acetoacetyl-S-CoA from the last four carbons of the fatty acid chain reacts with thiolase to give non-labelled acetyl-S-enzyme and this in turn reacts with labelled O acetyl-S-CoA from the pool to yield CH3—CO—CH,—C*—S—CoA. This is shown schemat- ically in Fig. 8 for the case of caproic acid. O CH,—CH,—CH.,—CO- O CHj—'CHo—CHg—'C- O -CHj—C —S—CoA (HS—Enz) O -S—Enz CH3—C —S—CoA (HS—CoA) CH3—CH,—CH.

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. Cell chemistry; a collection of papers dedicated to Otto Warburg on the occasion of his 70th birthday. Warburg, Otto Heinrich, 1883-; Biochemistry. 3o8 F. LYNEN, S. OCHOA VOL. 12 (1953) non-labelled acetoacetyl-S-CoA from the last four carbons of the fatty acid chain reacts with thiolase to give non-labelled acetyl-S-enzyme and this in turn reacts with labelled O acetyl-S-CoA from the pool to yield CH3—CO—CH,—C*—S—CoA. This is shown schemat- ically in Fig. 8 for the case of caproic acid. O CH,—CH,—CH.,—CO- O CHj—'CHo—CHg—'C- O -CHj—C —S—CoA (HS—Enz) O -S—Enz CH3—C —S—CoA (HS—CoA) CH3—CH,—CH.,—C—S—CoA HS—Enz. CH3—C CH3—CO—CHj—C *—S—Co A + HS—Enz l(H,0) CH3—CO—CH,—C*OOH + HS—CoA CH3—C*—S—Enz + HS—CoA O (CH3—C*—S—CoA O CH3—C*0—CH„—C*—S—CoA J (H^O) CH3—C*0—CH,—C*OOH + HS—CoA Fig. 8. Asymmetric labelling of acetoacetate from carboxyl-labelled caproic acid. ^-keto-redudasc. This enzyme catalyzes Reaction 6. The finding that o CH3—CO—CH2—C—S—C0A + DPNH + H+ ^ CH3—CHOH—CHg-C—S—C0A+DPN+ (6) the acetoacetyl-S-CoA analogue, S-acetoacetyl-N-acetyl thioethanolamine, was readily reduced by DPNH in the presence of enzyme solutions from various sources afforded a convenient assay for this enzyme. Employing this assay the enzyme was purified some 300-fold from sheep liver extracts by a procedure involving three steps: precipitation with ethanol, denaturation of inactive protein at 55°, and fractionation with am- monium sulf ate^^. The time course of the reaction in the optical test with varying concen- trations of the purified enzyme is shown in Fig. 9. The reaction is readily reversible but, at pH 7.35 with equimolccular amounts of DPNH and the acetoacetyl thioethanolamine derivative, it proceeds in the direction of reduction of the latter to the extent of 95 %. The equilibrium constant of the reaction (/iTe'q = (S-i3-hydroxybutyryl compound) (DPN+)/(S-