Its preferred substrate is linear 1,5-alpha-L-arabinan. The enzyme activity is progressively reduced as 1,5-alpha-chains become shorter or more highly substituted. Endohydrolysis of (1->5)-alpha-arabinofuranosidic linkages in (1->5)-arabinans. Glycan metabolism; L-arabinan degradation. By L-araban, arabinogalactan and L-arabitol. Belongs to the glycosyl hydrolase 43 family.