Cheesbrough T M, Kolattukudy P E
Ohio State Biotechnology Center, Ohio State University, Columbus 43210.
J Biol Chem. 1988 Feb 25;263(6):2738-43.
Alkanes are widely distributed in nature and impaired alkane synthesis was implicated in certain neurological disorders. However, the mechanism of synthesis of alkanes in animals is unknown. Our search to find a convenient animal tissue to study alkane biosynthesis resulted in the finding that the uropygial gland (a modified sebaceous gland) of the eared grebe (Podiceps nigricollis) produces large amounts of alkanes. These alkanes, which constitute 35-41% of the total lipid produced, are mainly C21, C23, C25, and C27 n-alkanes. Cell free homogenates of this tissue synthesized alkanes from both fatty acid and aldehyde in the absence of O2. Differential centrifugation of the homogenates indicated that this activity was located in the microsomal fraction. With isolated microsomes conversion of fatty acid to alkane required CoA, ATP, and NADH whereas conversion of an aldehyde to alkane did not require the addition of cofactors. That the final step in alkane synthesis is a decarbonylation was shown by the stoichiometric production of heptadecane and CO from octadecanal. CO was identified by adsorption to RhCl [(C6H6)3P]3 and oxidation of the trapped CO to CO2 by watergas shift reaction. The enzyme preparation also catalyzed incorporation of 14C from 14CO into octadecanal showing the reversible nature of the decarbonylase. This decarbonylase had a sharp pH optimum at 7.0, a Kapp of 180 microM and a V1/2 of 90 rho mol/min/mg protein for octadecanal. The enzyme was inhibited by the metal chelators EDTA, O-phenanthroline, and 8-hydroxyquinoline, but not by KCN. It was stimulated nearly 3-fold by 5 microM 2-mercaptoethanol and inhibited by the presence of O2. During the conversion of [1-3H]octadecanal to heptadecane, 3H was lost to water and 3H from 3H2O was incorporated into the alkane generated from unlabeled octadecanal. The mechanism of the decarbonylation and the nature of the enzyme remain to be elucidated.
烷烃在自然界中广泛分布，烷烃合成受损与某些神经系统疾病有关。然而，动物体内烷烃的合成机制尚不清楚。我们寻找一种方便的动物组织来研究烷烃生物合成，结果发现，角䴙䴘（Podiceps nigricollis）的尾脂腺（一种特化的皮脂腺）能产生大量烷烃。这些烷烃占总脂质产量的35 - 41%，主要是C21、C23、C25和C27正构烷烃。该组织的无细胞匀浆在无氧条件下能从脂肪酸和醛合成烷烃。匀浆的差速离心表明，这种活性位于微粒体部分。对于分离的微粒体，脂肪酸转化为烷烃需要辅酶A、ATP和NADH，而醛转化为烷烃则不需要添加辅因子。十八醛化学计量地生成十七烷和CO，表明烷烃合成的最后一步是脱羰反应。通过RhCl[(C6H6)3P]3吸附CO，并通过水煤气变换反应将捕获的CO氧化为CO2来鉴定CO。酶制剂还催化14CO中的14C掺入十八醛，表明脱羰酶具有可逆性。这种脱羰酶的最适pH值为7.0，十八醛的表观解离常数Kapp为180 μM，半最大反应速度V1/2为90 ρmol/min/mg蛋白。该酶受到金属螯合剂EDTA、邻菲罗啉和8 - 羟基喹啉的抑制，但不受KCN抑制。5 μM 2 - 巯基乙醇能使其活性提高近3倍，O2的存在则会抑制其活性。在[1 - 3H]十八醛转化为十七烷的过程中，3H会损失到水中，而3H2O中的3H会掺入未标记的十八醛生成的烷烃中。脱羰反应的机制和酶的性质仍有待阐明。
