Phospholipase D (PLD)-mediated transphosphatidylation enables enzymatic syntheses of phospholipids having various head groups. Due to the broad substrate specificity as well as the ease of preparation, PLDs of Streptomyces origins have been widely used for phospholipid production. For a long time, however, synthesis of phosphatidylinositol (PI) has not been realized with Streptomyces PLDs. The author’s research group has developed mutant PLDs having PI-synthesizing activity by protein engineering technique. This review introduces our recent results for the development of PI-synthesizing PLD and its application.

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The phospholipids metabolism has been known to play an important role in both animals and plants. In plants, phospholipase D (PLD) has been suggested to be involved in many plant cellular processes, such as signaling for stress and hormone responses. Plant PLDs are a family of heterologous enzymes and 12 PLD genes in arabidopsis and 17 ones in rice were reported. They can be classified into several types based on their gene architectures, sequence similarities, domain structures, and biochemical properties. To examine the physiological function of PLD in rice, we made knockdown plants for each PLD isoform by introducing gene-specific RNAi constructs. One of them, OsPLDβ1-knockdown plants showed the accumulation of reactive oxygen species in the absence of pathogen infection. RT-PCR and DNA microarray analyses revealed that the knockdown of OsPLDβ1 resulted in the up-/down-regulation of more than 1,400 genes, including the induction of defense-related genes such as PR protein genes and WRKY/ERF family transcription factor genes. HR-like cell death and phytoalexin production were also observed at a later phase of growth in the OsPLDβ1-knockdown plants. These results indicated that the OsPLDβ1-knockdown plants spontaneously activate the defense responses in the absence of pathogen infection.

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In this screening study, we report the characteristics and functions of the following novel enzymes isolated from actinomycetes: phospholipase (PL) A1, PLB, PLD, and glycerophosphocholine cholinephosphodiesterase. Investigations revealed that the characteristics of these enzymes differed from those of known phospholipases. These novel enzymes could have valuable applications as industrial enzymes, in material production, and in diagnostics. In this study, we produced a biodiesel fuel and glycero-3-phosphocholine by using PLA1 or PLB. In addition, PLA1 hydrolyzed the sn-2 acyl ester bond of phosphatidylethanolamine plasmalogen, indicating the potential for use as a diagnostic enzyme. We also observed novel substrate specificity for PLD and determined that phosphatidylcholine plasmalogen could be analyzed using PLB and PLD.

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Biological functions of lipids include (1) energy storage, (2) main structural components of cell membranes, and (3) signaling molecules such as lipid mediators. Lipid mediators represent a class of bioactive lipids that are produced locally through specific biosynthetic pathways in response to extracellular stimuli. Their biosynthesis is initiated by release of fatty acids and lysophospholipids from membrane phospholipids by phospholipase A₂ (PLA₂) enzymes. PLA₂s are involved in many physiological processes, and their dysregulations are often linked to various diseases such as inflammation, infertility, atherosclerosis, and cancer. In this article, we will give an overview of recent knowledge of several diseases in which particular lipid networks regulated by PLA₂s have critical roles.

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Phosphpipase A₁ (PLA₁) is an enzyme that cleaves fatty acids at the sn-1 position of glycerophospholipids. Although the PLA₁ activity has been widely detected in various organisms and tissues, its molecular details have been elusive. Recently several molecules that exhibit the PLA₁ activity have reported. Here we describe recent advance in the study of PLA₁.

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